Most Helpful Guy
While I personally believe everyone deserves a good slap from time to time, I know it isn't a popular belief, calling someone childish is not even close to a good reason to slap someone. Chances are he will strike you eventually for no good reason. All couples fight, and calling someone childish isn't that big of a deal.
If he at least felt bad about it, that would be something, but he doesn't. That is what concerns me. Now if she had tried to hurt him by mocking the fact that his mother was dying or something, that would be incredibly cruel and I think deserving of a good slap.