Lately, I have noticed a few unkind things on the playground too. It disgusts me that parents do not correct their children. Some parents are just oblivious. At the time, I was afraid to intervene because I did not want to be rude in front of the girl's mom (a friend of a friend). But, now after reflecting. I am going to speak up more. That way, my daughter (4yrs old) can learn the words (from me) about how to stick up for herself and handle the situation. I'll do this until I see her doing it for herself. I don't think it is an age thing but about learning a skill. Someone needs to correct the "bad" kids whether it's me or my daughter.
I think we have to be careful about what we say though - no character assassins. For example, I would never say to my daughter that the child is mean or bad. That is the same as name calling. I feel it is better to describe the action (not the person) as mean or we risk teaching our children to name call.
In the example of a girl blocking the slide so my daughter could not go down, I would say in a big voice and with a big smile "the playground is for everyone, it's not nice to block the slide, can you move so she can have a turn?" Later, I would speak to my daughter again about what words she can use to stick up for herself.
It's a tricky thing, but as parents, I think it is our responsibility to teach these kids. And, I am not going to worry about another mom being mad at me about something they should be doing for their children.