Was the Children’s Crusade really a willing venture led by a 12 year old, or is that merely folklore?

I have an earlier answer on the sources and historiography for the Children's Crusades, if you're interested--as with many medieval things, It's Complicated.

Something happened in 1212, and a not unimpressive something at that. Whether it's the Lyre chronicle's cursory note "People made processions" or the involved, tragic tale of Alberic of Troisfontaines, there's plenty of evidence for a mass of people on the move from north to south for religious purposes. And to be clear, north to south means hiking the Alps. The questions for scholars today are whether the participants were or were exclusively children, whether it counts as a crusade, and whether it should be singular or plural.

The middle question is a judgment call. Even Thomas Madden, who says "The Children's Crusade was not an army of children, and it was not a crusade," accepts the chroniclers' story that a mass of people, starting somewhere around Cologne and growing larger as they moved south, had in mind to re-recapture Jerusalem from Muslims. Kelly DeVries notes that it wasn't a crusade in the typical sense because they weren't an army, but (although he doesn't say it) I think Madden has something different in mind.

That is to say, the (so-called?) Children's Crusade was not a Crusade because it was neither started nor recognized by the Church. A crusade was defined by the vow taken by crusaders, but also by the declaration and indulgence offered by the papacy. The Children's Crusade lacked this official sanction. A reasonable comparison is the disastrous 1096 "People's Crusade" that stormed eastward across Europe while the official crusaders with Pope Urban II's approval were still packing their bags. We call it "Crusade," but it doesn't get the fancy designation of "First."

The more troublesome issue is the moniker Children's. There are two basic roots for the crusade consisting of children. First, some chronicle accounts mention that a boy less than twelve/less than ten named Nicholas was the leader. Second, the Latin pueri is indeed frequently used to describe the participants, sometimes accompanied by the female counterpart puellae. It is in fact reasonable to translate these as "boys and girls."

The difficulties, Peter Raedts argued, are that they didn't necessarily mean boys and girls, and that contemporary witnesses include many more categories of people. Social historians and historians of childhood note that "puer" can refer to unmarried men, especially agricultural day labourers. This use would fit in well with other evidence from the sources--parents trying to prevent their offspring from leaving since they'd be needed for field work at the time, and the frequent companion of pastores or shepherds as fellow crusaders. And while there's even a reference to breastfeeding infants making the journey, obviously these are accompanied by their mothers (including explicitly in the source) and some accounts note older people going as well.

I'm not sure "pastores" is as decisive as some scholars have treated it; I often read about children (including girls) working as shepherds or watching other livestock in hagiographies and court cases. And this leads into somewhat of a split in scholarship. The sources are clear on the participation of children as well as adults; the question is, did contemporaries understand the crusade as especially a movement of children, or especially a movement of peasants.

While rejecting Norman Cohn's view of the Children's Crusade as an anti-poverty protest, Peter Raedts argues that the terms used to describe the crusaders indicate that the chroniclers thought this crusade stood out because of the low social status of the participants. The complete omission to the presence of clergy despite mentions that the crusaders had official material objects of the Church along with them according to some chroniclers seems to suggest that there were in fact priests along the way, but they were omitted from the record in order to shape a view of it as the hardscrabble peasants'.

Gary Dickson, on the other hand, has done the most extensive study of the legend of the Children's Crusade, starting with contemporary sources. He notes that easily within 50 years of the events themselves, chroniclers focus on the age in unambiguous terms ("parvuli" or small ones). This suggests that the idea of children going along was what captured people's imaginations. (The later accounts are generally more extensive--people were talking about this. So we shouldn't see a sharp split between 1212 and 1300, but rather a repetition/surprisingly accurate game of Premodern Telephone.) I should, however, point out that Dickson argues against the 19th century romantic image of all small children participants. (The construction of the myth, which he calls the unpronounceable mythistory, is much of the focus of his book.)

The third question is whether Children's Crusade should be singular or plural. There is definitely the movement from Cologne to Italy. But did a simultaneous event arise in France near Vendome, and was it connected, and was it aimed at going to Paris to inspire the king to crusade or were the participants vowed towards Jerusalem? A mass procession from Vendome towards Paris is generally accepted. Dickson argues hard for a connection (despite, as he notes, the lack of "direct evidence"). To Raedts, on the other hand, the contemporary evidence is focused on a political-religious pilgrimage of sorts to Paris; it's only later that sources develop the idea of the group as a "crusade" or start to mention Jerusalem. Again, both scholars admit points for and against their particular view. (I personally would like to see more analysis of the later sources in light of debates over spiritual poverty and the priest versus friar rivalry, but spitballing right now it would probably just result in more fuel for both sides.)

The wider problem that both Raedts and Dickson are grappling with in trying to deduce a possible link between the German and French movements, and I would argue debating whether this should be the Children's Crusade or the Peasants' Crusade, is--surprise, surprise--historical context. 1212 is right on the cusp of 1215, when the Fourth Lateran Council will commit the Church to the instruction and pastoral care of lay people. This doesn't come out of nowhere--it's the result of Church officials recognizing that over the course of the twelfth century, the proletariat Got Religion and are going to exercise it with or without the Church.

In addition to the proliferation of new monastic orders (indeed, the entire concept of different monastic orders) in the 12th century, which arose from the nobility and new urban aristocracy, there were more "popular" movements of people coming together to practice and preach Christianity. You'd think this would be awesome for the Church, but prelates tended to see these movements as a threat, since they initially operated outside clerical hierarchy. The Waldensians (who believed in lay preaching) were condemned as heretics; the Franciscans, who were started by a lay man preaching, quickly had their historiography overwritten as Francis being a deacon (this is why Francis is always preaching to animals in his iconography; he wasn't actually a cleric); the all-woman beguines steered a perilous course between acceptance and suppression.

A frequent feature of these groups was a commitment to poverty as a spiritual ideal. They weren't calling for peasants seizing the means of production or the end of a nobility, but instead insisting that "the poor" had a spiritual role to play where rich people had failed, and an insistence on poverty as a way to imitate Christ. We should read the chroniclers' accounts through this lens of poverty as an ideal (not one they particular wished to claim for themselves, naturally), as well as the actual fact of poor and middle class people claiming an active and formative religious role for themselves.

So something absolutely happened in 1212, something emblematic of the surge in religiosity sweeping over Western Europe at the time. Although many elements of the "traditional" (Victorian medievalist) view have been decisively overturned, whether it's accurate or badhistory to call it Children's or Crusade isn't an entirely settled question.