As I understand it, explicit concern over the psychological (as opposed to physical) impact of child abuse/sexual abuse is a 20th century development; we have some great history of psych flairs who would be able to tell you more about that. However, this is an interesting question because OP has specifically asked about sexual practices that were not considered abuse in a particular historical context.

So, with apologies for straying outside my usual focus:

Late antique Jewish moral writing, and then derived from that early Christian authors, comes down very harsh on Greco-Roman institutionalized traditions of pederasty/pedophilia (paiderasteō). The idea of corruption of the boys involved runs deep.

Philo of Alexandria, De specialibus legibus III.37-42, explains:

Moreover, another evil, much greater than that which we have already mentioned, has made its way among and been let loose upon cities, namely, the love of boys, which formerly was accounted a great infamy even to be spoken of, but which sin is a subject of boasting not only to those who practise it, but even to those who suffer it, and who, being accustomed to bearing the affliction of being treated like women, waste away as to both their souls and bodies, not bearing about them a single spark of a manly character to be kindled into a flame

Philo's specific criticism is that being the victim of Greek pederasty makes boys accept and want the practice. In another text, De contemplativa vita, he more obliquely criticizes how pederasts use and discard the youngest boys.

Following this line of corruption, early Greek Christian texts will introduce the word paidophthoreō to label the same practice. But rather than the eros/carnal love connotations of paiderasteō, John Martens argues, paidophthoreō invokes a web of meaning of destroy or corrupt. The Didache, a very early Christian text, places paidophthoreō as a crime next to infanticide: the utter destruction of children.

Some sensitivity is mandated here. Philo's criticism, after all, is that the victims of pederasty (he uses the older Greek term, although he condemns the practice; there is only one Jewish source that uses paidophthoreō) grow up to be too feminine, and heavens forbid we should accept ancient and medieval ideas of gender boundaries as trans-historical, Platonic ideals. I'm not comfortable with the idea of wholesale dismissing gender performance as 100% the result of sexual abuse, no question, move along.

However, I think there is a good argument to be made based on contemporary and later traditions that the idea of dealing with sexual abuse/pedophilia by striving to accommodate it, mentally, was implicitly and obliquely recognized by late antique and medieval authors. I'm no psychologist and it's important to remember that the modern idea of psychology, mental illnesses, and even emotions are culturally constructed, but it seems to be that modern research finds some resonance in this idea.

So I want to turn to a different type of case, fictional narratives of incest. Medieval sources really don't like to discuss sexual abuse of children directly, but as Kathryn Gravdal points out, incest stories present a way around or through the silence by keeping the focus elsewhere. She differentiates between "canonical incest," or degrees of consanguinity for marriage purposes, and incest within the nuclear family. Specifically, we have stories of mother-son and father-daughter incest which are, always, turned towards an ultimately didactic purpose. What I want to highlight here are the very different standard portrayals of those types of incest in hagiographies, narrative poems, and exempla (moral lessons).

The prototypical mother-son incest story is, I would argue, the heir of the "corruption" critique. In these, the mother is a new widow who sees in her child the spitting image of her dead husband. She initiates the sexually abusive relationship; the boy grows up to fall in love with her. Again, the idea of aligning oneself/accepting the situation as a way to deal, cognitively, finds resonance both in Philo's writing and in modern research.

The stories of father-daughter incest are very different, certainly also reflecting gender norms. Here, the father is portrayed as a predator. As with stories of saintly women fleeing arranged marriages, the girls disfigure themselves--even to the point of amputating a hand--and flee their homes. The stories recount their suffering and hardships in the world. These are external consequences, of course, not "internal" ones--don't expect to find long descriptions of PTSD-type nightmares or flashbacks. However, these texts (unlike the saints who disfigure themselves to flee marriage, who usually wind up martyred) generally have more upbeat endings. The daughters, after massive suffering, will often wind their way to a husband and children of their own. There is room to see, given the medieval refusal to confront certain subjects directly, these stories as a sort of externalization of internal processes. (A literary topos that medieval writers loved.)

Incest is a different case than institutionalized pederasty or early marriage, obviously. However, the parallels between the father-daughter incest narratives and the standard flight from early marriage stories are a really striking overlap that, I argue, justifies an extrapolation from specifically incest to forced sexual activity as a child in other circumstances as well.

So with all the care and sensitivity needed to reconstruct premodern emotions, psychology, and sexual norms, plus some careful reading of and through sources, I would argue that we can see both the emotional effects of childhood sexual abuse and an implicit, oblique awareness of them.

http://www.hawaii.edu/PCSS/biblio/articles/2000to2004/2004-sexual-behavior-in-pre-contact-hawaii.html

I'm not a historian, but in the interest of representing non-Western cultures, I thought I'd introduce this academic paper I came across on reddit a while ago.

The paper, which discusses the sex-positive culture of pre-contact Hawaii and Oceania, observes that in the context of a non-Judeo-Christian society with no sociocultural/religious grounds to imbue sex with guilt, sexual relations between adults and nonadults of both sexes were apparently an open and frequent occurrence. There is no mention of trauma or psychological damage on the part of the children, who are instead depicted as being active participants in those sexual acts, as well as growing to be fully functioning members of their community.

I would love if any historians and anthropologists could comment as to the reliability of this paper as a source, as well as any other insights that could shed light on the veracity of the paper's claims.

What ages do you specifically mean by "children"?

Supplementary Q: how does the treatment of girls and women in such societies affect the quantity and quality of historical evidence available to address this question? In terms of their ability to be heard and leave a record of their experience but also the effects of internalising oppressive traditions?

It strikes me that /r/anthropology might have some additional insights into this question.

As a follow up, the first novel written, the Tale of Genji, involves the main character being madly in love with a 10 year old. He essentially grooms her. Her being 10 is not mentioned as a significant point, but rather offhandedly. I cant tell if its because at the time Japanese culture had very strict ways in which to talk about nobility (for example every character is referred to as position, never by name, even when their position changes) that prevented them from saying "He was a creepy pedophile". But my question is do we have evidence of this type of thing being common in East Asian culture? Was there a tradition of pederasty, or of child brides? Were children seemingly okay with it or would custom have prevented any writing about it?

Edit: misread the age, it was a 10 year age difference...but he was 18 and she was 8

similarly, is there any documented trauma on the part of eunuchs, particularly those that rose to prominence in Muslim societies?

I can answer this from the more general perspective of psychology and psychopathology in history, rather than specifically with the incidence and consequence of sexual practices in childhood/adolescence, because my expertise is the history of psychology/psychiatry/psychopathology and because I was a practicing therapist. However, the example I use may well be evidence of the psychological trauma OP is asking about. That being said, mods please feel free to delete this answer if it isn't probative enough.

Not all mental illnesses (let's call them phenomena for our purposes) are seen in all cultures at all times and the ones for which we have evidence of clear biological roots/contribution that DO appear in all cultures at all times are not always viewed as disorders or even seen negatively (symptoms of psychosis may be viewed as spiritual gifts, etc. (Not surprisingly, religion is often the lens through which mental phenomena are viewed and understood.) So cultural relativism becomes hugely important in disorders that are NOT always recognized as negative in all cultures at all times (and there are plenty of disorders that are specifically bound by time and culture). Let's look at one example a psychopathology that has been viewed differently at different times - anorexia and its partner-in-crime OCD. For our purposes, we'll call them phenomena rather than disorders.

Joan Jacobs Brumberg wrote an amazing book on the history of anorexia nervosa and the phenomenon of self-denial in women. We know anorexia nervosa today as one of a set of eating disorders and extreme self-denial currently falls under the category of OCD (the term orthorexia has been used to describe extreme self-denial but it is not yet clinically accepted terminology. I use it here because I find it a handy term). We view them as mental disorders and treat them as such. But as Jacob Brumburg demonstrates so articulately in Fasting Girls, these two phenomena are not at all new, and she starts her documentation of them in Europe in the 16th century. At the time, they are viewed through the lens of Christianity and are considered examples of extreme religious devotion and practice. (Jacob Brumburg discusses these phenomena among women, but religious asceticism is not unique to women). She then traces anorexia/extreme denial through the years as it shifted away from religious asceticism and became tied to social standing and wealth, cultural ideals about women, women in the entertainment industry and even to the industrial and scientific revolution. It is only in the 20th century that these phenomena begin to be classed as medical and psychological disorders.

So even with phenomena that are recorded very clearly in particular cultures over a period of centuries, they are not always considered to be Bad Things like they are today. And this applies to cultural behaviors as well. They need the same relativistic viewpoint.

That being said, the example that I specifically have described may in fact be an example of what the OP is looking for. There is clear scientific consensus that anorexia, orthorexia and other forms of extreme self-denial are often, in our current society, the devastating result of childhood sexual trauma. While attitudes about the age boundaries of childhood and adolescence may have changed, the response to what we consider now to be sexual trauma may have been the same. One could make the argument that the behaviors of extreme self-denial seen in centuries past were rooted in the same causative behavioral phenomenon. I don't have any specific examples at hand (maybe /u/sunagainstgold can help here), but convents were one way of escaping sexual trauma, so it may be that if there were psychological repercussions from significant youth at the time of first sexual contact/marriage were in fact experienced as trauma, religious asceticism in convents and the continued existence of anorexia/OCD/orthorexia in women throughout the last few centuries could well be the evidence.

Ok people,

let me make a few things very, very clear here:

This question is a valid historical questions that is absolutely valid to ask about here. This is AskHistorians after all and the field of sexual and gender history are very interesting and great fields.

What will not fly here is just trotting out isolated historical examples of children being taken as lovers without providing historical context and fitting them in overarching historical narrative. As our rules so clearly state:

Answers in this subreddit are expected to be of a level that historians would provide: comprehensive and informative. As such, all answers will be assessed against the standards of Historiography and Historical Method. You should cite or quote sources where possible. A good answer will go further than a simple short sentence. As described here, "good answers aren't good just because they are right – they are good because they explain. In your answers, you should seek not just to be right, but to explain."

And one other thing that will absolutely not fly here is either stupid jokes about child rape, hurling insults at OP, or this weird age of consent lawyering that has popped up. This is a historical discussion, not one about the age of consent in the current day US or any other modern-day country. If you can't keep the discussion historical, don't post here.

This is your one time warning. Not heeding it will be met with the appropriate consequences.

Edit: Also, the next person asking about "not stupid jokes about child rape" will get a permaban. This is not a topic to kid around with or be a smartass about.

Thank you for your answer anyway, it gave me food for thought!