How to confront adults who insult children?

What is a polite way to let an adult know they have said something that isn't appropriate to a child? I've been in situations where I've heard adults around me, in a social situation, tell children (not their own) that they are stupid or an idiot...

and stuff. I think this is very rude and harmful to the children. Is there a way to tell them this isn't proper behaviour without coming off as offensive?

I don't think there is a nice way to do this, unfortunately. In my social circle, I'm usually the paranoid dad and I've lectured many other parents about the way their kids were behaving, acting, language, etc, etc.

Every time, without fail, they look at me like I'm the crazy parent. However, I firmly believe that you can't just sit there observe when people show poor examples OR poor parenting skills It's something (as a parent) I firmly believe that you should take up with the other parent. Maybe you get through and maybe you don't, but the important thing is to try.

Sometimes parents are stressed up and frustrated and say it out of anger. It's something else if the child is being treated like that daily like being isolated and called that.. if it gets to that level thats when it gets confronted i guess..