I've only been in this sort of situation once, where I really WANTED to do something, but didn't have a clue WHAT. For the most part, people who are just spanking their kids (open-handed, on the bottom) are NOT "abusers," in that they are just doing the best they know how, and they are unlikely to punish their child if someone approaches them about the spanking. I was spanked, and almost everyone around here does it. MOST of them may be offended that you would criticize their parenting, but they wouldn't take it out on their child. It can be hard to know, though, can't it?
The one time I had a similar experience was in WalMart, alas. This woman was cussing at her son -- he was maybe 5 or so. I was irritated by this, but I didn't say anything. This woman was very sour and hateful-looking... I was sure this wasn't just a stressed-out reaction. She gave me the creeps. I met up with DH on another aisle and told him about the woman; if we were quiet we could hear her yelling, wherever she was.
A little while later we met up on another aisle. The woman paused to pick something up and talk to the other woman who was with her, and the little boy walked in front of the buggy (or cart... we call them buggies here) to look at something. As soon as the woman (mom, I assume) was ready to go, she said, "Get out of the way!" After about 2 seconds, when he didn't move, she ran the buggy into him. I was so appalled, and I will always, always be angry with myself for not intervening. I felt sick to my stomach; the other woman didn't say anything at all. I told myself I would tell an employee about it when I saw one, but there weren't any around. I know I was just sort of rationalizing why I "couldn't" help. That was the last we saw of them, until we were out in the parking lot. I was at the soda machines, and the women and boy were at their car. It was a station-wagon type vehicle; the boy was sitting in the back seat, while the women put the groceries into the very back of the car. He turned around to ask them something, and the mom (?) started yelling at him to "turn your @$$ around and sit down! You always have to make everything so difficult! Why? God, I wish I'd never f***ing had you!"
I just didn't know what to do...
I knew then that the people would be gone before I could get help, and I felt positive that my own intervention would have just meant more abuse for the child. This was probably 2 years ago. I still think about that day. I am determined never to let anything like that slide ever again, but it doesn't make me feel any better about letting that boy go home with that terrible woman.