I see parents who I think are mistreating their kids all the damn time. One of the most painful things, for me, is seeing grown adults screaming at toddlers to keep up with them, and yanking their arms so hard the kid lifts off the ground. I also hate seeing parents who are clearly stressed and have reached that point where everything they say becomes a seething scream at their children.
My response... nothing. Like most people, I don't really want to be involved. If I saw something too outrageous, I'd probably step in, but that's never happened. Hell, I'd probably be more inclined to step in if I saw them mistreating their dog in such a way, which is pretty sad, I guess.
I know this answer is pretty pathetic, but it's the truthful way 99.9% of us respond, and I just wanted to give you an honest answer.