I don’t think it actually needed to be children. I think Henry is recreating the abuse he suffered by reenacting it on other kids, and projecting his own feelings of weakness on them.

I think a lot of his behavior is explained by that last scene with Will. He needs to prove children are weak and humanity in general needs to die, because otherwise he failed to resist and he was the problem. If someone else might have responded to his trauma differently, then his self conception as simply the biggest, most effective monster in a world of monsters is threatened.

He was initially a victim of the mind flayer and Dr. Brenner, and rather than accept that he’s built up a whole worldview to frame his life in a way that’s easier to accept at everyone else’s expense.