oh, that's awful. I've seen a few things like this. I hate it when I see adults swear at little ones, but seeing them hit kids is disgusting. I don't believe in smacking at all. I was smacked as a child and it's just obscene violence. Made me resent my dad LOADS and it took me a decade to resolve these feelings, despite having many good times with him, the violence overshadowed it. Smacking is intended to get the feelings of an adult's frustration out, not to actually deal with a childs behaviour, teach them how to actually behave, or have self control in any way. If the adult has no self control, how can the kid be expected to grow up with any? I think it sends totally the wrong message.
If I didn't feel comfortable approaching the parent or contacting the cops, I'd call on the centre management. I think you did the right thing, next time, maybe ask to speak to a manager, as the have a more vested interest in fixing these probs in their store than a checkout attendant.