I myself have seen abusive situations. I try to be a presence without saying anything. It did seem to make the parent self conscious enough to hurry them along. They could sense my disapproval without saying anything.
Problem is there are still a lot of parents that were brought up with spanking as a coping mechanism. It is not that I don't have any guilt, it is that I know how it hurts the persons who hurts another as much as it hurts the child. They are both victims of negative thinking.
I think it is risky trying to help cope with other people's kids. The parent can turn into a snarling animal. No, too risky for me. And I am not perfect either.