In 1212, two groups involving children embarked on crusades to the Holy Land. They are generally lumped together and known as the Children's Cruade. They were not true crusades in the sense that the pope did not call for a crusade and nor did he sanction it (or them).
One group (from Germany) was led by a shepherd called Nicholas. They had been inspired, according to History Guide, by an earlier group from France. When they arrived in Rome, they were told by Pope Innocent III to return home.
The earlier group, also led by a shepherd, Stephan of Cloyes, got to Marseilles where they boarded ships under false pretenses - they were sold into slavery. They had been advised by the French King Philip II to return home.
According to Brittanica.com, the participants of at least one of these groups took the Crusader's vow, and this vow was recognized as valid by the church, but it is not clear whether this recognition was at the time of the crusade or at some later point.
I can't find any reference to any response of the Pope to this first group. Was there any, or did the pope just react to the second group (from Germany) because they turned up in the Papal States? Also, did the church take any action to prevent further groups of 'unofficial' crusades involving children?