Depends on the definition of "sexual abuse." If you're referencing the contemporary problem of (Catholic) sexual abuse of children, I haven't come across any accounts in my research. (I work on late sixteenth, seventeenth and early eighteenth centuries in Central/Eastern Europe.) Starting in the later sixteenth century, bishops tended to be increasingly mindful of clerical shortcomings in their respective dioceses, especially in response to Protestant accusations of Catholic clerical (sexual) laxity. By the later sixteenth century, episcopal visitation records and so-called letters of supplication (from parishioners or village communes) became ever more frequent and recording of professional, spiritual, financial or moral clerical deficiencies became more routine and standardized. Throughout this period, I have come across numerous examples of parish priests being sexually involved with their female parishioners, both married and single. Some were listed as young as 14 or 15, but this was hardly considered child abuse in the period. Reports of same-sex indiscretions were rarer still, in my experience, usually confined to monastics (in this case, both male and female). According to ecclesiastical court records, punishments for clerical transgressors would include fines, sometimes imprisonment in the cathedral dungeon, sometimes outright removal from a benefice. In answer to your question, this isn't to say that instances of child sexual abuse did not take place. However, given their absence in the source base, it's fair to say that they were much more rare than the garden variety transgression of seducing the miller's daughter.

Some secondary materials: Stanisław Litak, Parafie w Rzeczypospolitej w XVI-XVIII wieku. Lublin: Wydawnictwo KUL, 2004; R. Po Chia Hsia, The World of Catholic Renewal, 1540-1770 (New Approaches to European History). Cambridge, UK: Cambridge University Press, 2005; Jean Delumeau, Catholicism Between Luther and Voltaire: A New View of the Counter-Reformation, London: Burns & Oates, 1977; Marc R. Forster, The Counter-Reformation in the Villages: Religion and Reform in the Bishopric of Speyer, 1560-1720, Ithaca, NY: Cornell University Press, 1992

I can at least point to a “massive” phenomenon (or, at least, a phenomenon that incurred a massive reprobation): the case of sollicitatio ad turpia, i.e. the practice of “soliciting” (whether by the mean of spiritual blackmailing or by force) sexual relations in confessional. The first attested cases are linked (at least, to the best of my very limited knowledge) with the rise of confession as a mean of social control, from the 13th century on. The latin expression that I have mentioned was actually forged much later: it was created in the 16th century, partly to deal with the fact that the Reformation had used solicitation as an example of the corruption of the Church. This practice is best documented in the Spanish world, thanks to the efficiency of the Inquisition; some of the cases that it recorded include child abuse, of both boys and girls. However, I do not think that child abuse was perceived as “particularly” (by that I mean “needing to be addressed in particular”) problematic until recently.

(I think that the earliest source addressing specifically the issue of sexual intercourse between clergymen and young boys that I have come across may be a criticism of Jesuit schools in 18th century France; but I cannot remember the exact title of the book. If someone has a clue…)

Link to the earlier thread on this question.

The other comment mentioning the "13th century" is studiously vague but, if you go looking for more info, you'll see mention of the 1227 Council of Treves. First, that's the same as Trier and, second, it looks like 1227 was probably an error for 1277. [Blattau's collection of its acts (Latin).]

Similarly, regarding what definitions you're using, Wiki's links are dead but here is the actual CUNY report on abuse within the church between 1950 and 2002. The majority of incidents were touching and 25% of victims were aged 15-17; in the past, if exposed, either would have been condemned but not necessarily considered sexual abuse of children.