In the summer of 1212, thousands of divinely inspired young people from across Catholic France and Germany took off to liberate Jerusalem from the Muslims. None made it to the Holy Land. Many died along the way or were sold into slavery. As military campaigns go, the Children’s Crusade was a disaster. Yet environmental activists and politicians are adopting the same “a child shall lead them” strategy to push their climate change agenda and its latest incarnation, the Green New Deal.

Youth-oriented climate groups have proliferated...