From the purely short-term-consequentialist perspective, and assuming the unrealistic scenario in which I knew at that time that this baby would become Hitler unless I killed him right there, and assuming I could get away with it - it would make sense to kill him. But these are some strong assumptions to make.
Even worse, the purely short-term-consequentialist perspective is extremely limited. First, what would it do to my psychology, to kill an (at the moment) innocent baby with my own hands? I might have ended up messed up for the rest of my life, and the fact that I have prevented some terrible atrocities would not have made up for that. Second, what kind of person would I become if I decided that killing someone harmless at the moment in the name of the "greater good" is justifiable?
Other minor problems have not been mentioned by anyone I know, but they are still there. How do I know that, by killing Hitler, I am not condemning Germany to someone else using the chaos of 1929-1933 to further his agenda - perhaps, someone far worse than Hitler? How do I know that the Soviets will not overtake Germany and the entire continent subsequently, causing even worse atrocities than what the Germans under Hitler did? And so on, and so on, and so on.
I think that people who instinctively jump to the positive answer - "Of course I would kill a baby Hitler" - have not thought the matter through well enough. Most choices in life are not just about one specific outcome: they propagate through one's entire life, even the entire world sometimes. Something as minor as indulging in a pizza at a party when being strongly committed to a healthy diet may become the difference between sticking to the diet, and remaining a sad chubby boy for the rest of one's life. Something as major as killing a baby? Well...