Yeah, I've seen stuff like that too. It pisses me off as the kids are pretty innocent and look up to their parents, who should know better.
I find it hard to ignore and my feelings usually show very quickly, a 6 foot 250pound+ turning around abruptly glaring at you usually shuts them up pretty quick. I have never actually said anything though as it would probably make the situation worse.
One of my mates used to run a community centre taking care off disadvantaged kids and used to see some of them come in with bruises, obviously from their parents. One day when a drunken dad came in to this guys centre and started to shout to his son "move your f**kin as" , we're going home".
My mate (he's a bit shorter them me but built the same way) lost it and grabbed the dad by the collar, took him outside (slamming him violently into every wall and door on the way ) and told him
"Do not ever, and I mean ever swear in in my center again and if I ever see any more bruises on your son again, I'll come and break every f**kin bone in your body".
That bloke never came to the center again and more importantly, there was no more bruises on his son after that.
Apart from that one time though, he tried not to interfere with the parents. He told me
some of them were real low life and would take it out on the kids when they got home.