I found the answer to the question about a Children’s Crusade to be rather lacking, unfortunately.
“Were they actually children, though? Doubtful. Scholars argue that in medieval Latin the word puer, child or boy, could also be applied to a young man, especially if he was landless or otherwise of low standing–and folks with nothing to lose are always up for a little adventure.”
Or Puer could mean “child”, exactly as the popular version goes. Sometimes a spade IS a spade. The medieval conception of a child was radically different from that of our modern concepts of the preciousness, individuality, and vulnerability of childhood. Medieval children were really treated as small adults, and very early in life handed responsibilities and jobs that we wouldn’t consider safe for children anymore. In many cases, I daresay that they were little more than a regenerable labor force. If you had 16 children and couldn’t feed them all, to send one or two off to do God’s will might both score you some Heaven points, as well as making survival for the rest a little more possible. In this case too, it would be logical to send the YOUNGEST, unable to contribute meaningful amounts of labor to the running of the household. Odds are, this wasn’t “ALL CHILDREN” or “ALL LANDLESS YOUNG MEN” but a mix of both.
Further, you state "The distances traveled (roughly 35 kilometers a day) strongly suggest this wasn’t the T-ball crowd. " You’re more right than you know. I suspect that even young kids in the middle ages were FAR sturdier and inured to hardship, as well as being far, far more used to walking long distances to get somewhere. They weren’t the lazy nintendo-playing corpulent children of 2004 - they were the hardy survivors of a rather nasty Darwinian environment in which half of their siblings died before they were 2. 20 miles a day, over the well-travelled roads of Germany doesn’t sound like that tough a proposition for kids of that era.