> As my history professor put it, "going to the Holy Land was an incredibly expensive endeavour reserved for a very few. There's no way anyone was going to waste it on young, inexperienced children".

While a Crusade made up primarily of children is not realistic. The 'People's Crusade' was a thing. During the first Crusade 40 thousand or so poor peasants and various religious fanatics including many women and children departed Northern France a couple of months before the 'proper' army left.

It wasn't pretty. Along the way they murdered thousands of Jews, sacked or attempted to sack multiple cities and most of those who survived the journey were eventually massacred by the Turks (according to some that's one of the reasons why the Turks were so ill prepared when the actual Crusader army arrived, they expected that all westerners were poorly armed, disorganized and had no understanding or proper warfare... )