Perhaps Im just a cautious person at heart, but given the ability to change anything in the past, I would change nothing. Especially not something so important.
WWII was a monumental event in human history. If it never happened... There are too many events to even comprehend how the world would be different. Even my country alone, the US, would be unrecognizable without WWII.
Even given the horror of everything that happened, I could not in good conscience plunge the world into the deep end with absolutely no idea how it would turn out.
The question you're really asking, I presume, is, "Would you kill a baby if you knew, for a fact, that it would make the world a better place or, at least, prevent the world from becoming a horrible place?"
In which case I would say, if killing the baby were my only choice in this hypothetical, then yeah, I guess so. Assuming I was omniscient and knew exactly what would happen if I didn't kill it.