Children learn things since childhood and if they get support for their wrong deeds then I think it's the fault of the parents. It's the parent's responsibility to correct them in such a situation rather than avoiding it saying they are just a kid. I think that person did the right thing slapping the kid and I think I would dare to do the same as I don't tolerate such a thing.
As in ehn... using the banner of kids to enable them. It's just crazy.