Earlier quoted context omitted.
You don't know that the child wouldn't have grown up Hitler. The argument seems similar - not exactly, but similar - to the question "if global warming is real, why is it snowing here". A function describing the maximum global integration over happiness could very well contain many local minima.
Yes, but an omnipotent God could presumably just make Hitlers not exist. The argument rests on the assumption that there are hidden dependencies in the laws of the universe, such that it was logically necessary that Hitler (or insert whatever other evil here) had to exist to make the best possible world. That's hard to swallow.
More likely they were operating under significant constraints…