slappers

Last weekend, I was waiting in queue in the inappropriately named express lane at the stupormarket when I saw something mildly disturbing.
A couple of early teen kids, a boy and a girl, were generally making some noise, play fighting with each other, when their mother turned on them and casually, almost passively, slapped them hard on the head alternately, one, two, one, two, as they cowered away from her palm.
As soon as she turned away the children looked up, looked at each other and then with massive smiles started rapidly slapping each other in the head, too.
Now where did they get that idea from?
And why do I never tell parents off for abusing their children?