ummmm - //Andy - I was only comparing the pain, not the reasons. //
I could probably walk out into the street and find a total stranger, and reasonably assume that a punch in the face would not hurt him. It wouldn't benefit him in any way either, but that's beside the point.
I would not be able to justify my action in court on the basis that the man appeared to suffer no pain - it's still assault on a stranger for no good reason.
But babies are not able to consent, or resist, so the idea that it may not hurt is really beside the point.
Who would let their child find out the hard way? I wouldn't.