I just witnessed this the other day. I was sitting in the waiting room of my doctors office, and this mother and child, were sitting across from me smart-assing one another. She'd pick on him for doing something silly or stupid (or that she didn't want him doing, like clicking his pen/fidgeting), and he'd say something smart-assy back. This went back and forth for a while before he ended up slapping her. Then she slapped him back. And they repeated slapping eachother back and forth. This continued for a really awkward amount of time.

At some point the child started saying: "Stop hitting me!" while holding back tears. There was no receptionist on duty, and I wasn't sure what to do, either. I feel awful because I ended up just leaving the room, hoping the doctor would come out and see what was happening and intervene. Ugh...Maybe I should've said something?