In addition to a really horrible event in the Middle Ages, the Children's Crusade was a not-so-horrible (though perhaps questionable) part of the 1963 Birmingham Campaign of the American Civil Rights Movement, during which over a thousand school children marched downtown to talk to the mayor, many of them getting arrested as a result.
In 1963, the Sixteenth Street Baptist Church, with Martin Luther King, Jr. leading, launched a campaign to end segregation in Birmingham, Alabama. Some desegregation was being forced by the courts, but when, for example, public parks could no longer be segregated, the city opted to close them rather than allow integration. To force the city to negotiate and ultimately eliminate segregation, the protesters would non-violently break the laws they wanted changed, accepting the inevitable arrest. Well into the overarching campaign, a protest doing just this using school children was organized by Rev. James Bevel. On the first day of the campaign, over a thousand African American children left school, gathered at the Sixth Street Baptist Church, and then marched on downtown to
talk to the mayor get arrested, some reportedly as young as eight years old.
It kind of makes you look bad when your protesters peacefully accept arrest, and doubly so when they're not scruffy ruffians, but nicely-dressed youth as compliant as can be as you shuffle them off to jail by the hundreds. Some of the most famous Civil Rights Movement images occurred during this campaign. You may have seen the one of a police dog attacking a young teen, or the firehose being turned on a bunch of kids. Better a fire hose than rubber bullets, but it was still enough to anger the general public.
Naturally, the kids were released--at least some of them that evening or the following day. Annnnd they marched again. The Children's Crusade lasted three days and actually succeeded in overfilling the Birmingham jails--one of the original goals of the broader Birmingham Campaign--though since these weren't hardened criminals the police just used a stadium as a holding pen for some.
This was perhaps the protest that really ended the Birmingham Campaign--on May 10, 1963, local officials agreed to end segregation downtown stores and release all jailed protesters if the protests ended. A few days later, the Board of Education said any student participating in these protests would be suspended or expelled. This went to court and the protesters quickly won.
Though effective, Malcolm X and others objected to putting children on the firing line. There's definitely a case to be made that it was irresponsible to allow young children to participate in the protests, but at least for those old enough to understand and support the cause, there's additionally a case to be made for letting them take a stand for their own freedom.