I believe there has to be some truth in it, because it has been around for a long time, and it sounds like something some of the religious fanatics of the middle ages would have done.
In the last couple of decades, there were other stories about women going to Muslim countries and getting raped and murdered. A Woman going to Haiti after the earthquake who got raped, even though she explained to them that she was there to "help them".
I've heard stories from people who had been in the peace corps.
https://en.wikipedia.org/wiki/Children%27s_Crusade
Your recollection is good. Didn't end well for the kids unfortunately.
The ones that went to Muslim countries? Some were rebelling a found a really stupid way to do it. Those two Austrian teens wound up dead.