To my knowledge, Klarion the Witch Boy is the only recurring child supervillain in either DC or Marvel comics, so he’s really on of my favorites. I am sure there are more here and there, but Klarion has seen a lot of attention, appearing on Young Justice, Batman the Animated series, and Justice League Action. He also got a shortlived comic series, and he’s also teamed up with the good guys when the need arises. DC did age him up to like 16 or 17 recently, but in most other media he still takes the form of a child. A petulant, spoiled child with a mastery of chaos magic. What I’ve always like about him, is his “alignment” was never evil. He’s more chaotic neutral, more of a force of nature. He just is and he just does. That’s a lot of fun for a villain.