The so-called “Children’s Crusade” is a story that in the early 13th century a wandering child preacher gathered thousands of other children to march/sail with him to the Holy Land and convert the Muslims who occupied it. There are several supposed variations of the story but in the end they all end up either dead or enslaved.
The story is generally considered apocryphal by the majority of scholars of the era. There are two separate events that happened around that time that included teenagers but adults were also among both groups, neither was a crusade solely of children. During this period in history there were several “popular crusades” that were organized by various individuals or groups within Europe and were not sanctioned by the Pope.