Firstly, I've noticed a lot of Hitler references lately and this is confusing to me.
Secondly, the supermarket should not have denied the child a birthday cake because his name is Hitler, they're being *****ebags and it's entirely not reasonable. The person of Adolf Hitler was a jerk but that doesn't mean that his name is automatically offensive. This would be like me disliking Barack Obama because his name is very close to Baraka and he made my life a living hell when Mortal Kombat first came to a local arcade.
Thirdly, I don't think that naming your child after one of your heroes (no matter how much of a jerk someone else thinks of that hero) should be considered child abuse. For instance I would never name my Mohammad because I think that Mohammad is a poor idol to choose from when naming a child but that doesn't give me the right to impose naming laws on someone else's kids. Obviously these people like Hitler and it is their right to like Hitler and if that means naming their kid Adolf Hitler then sobeit.
Lastly, hopefully this kid will get picked on and hate his parents so much that he marries a bisexual Jewish man. Seriously though there's no reason that the kid is going to catch that much more flak than any other kid in school. Everyone gets picked on your name rhymes with Cryin? Your name rhymes with Puke? By the time he and his peers are old enough to know who Adolf Hitler is he'll probably be going by a totally different name.