It's a difficult situation to be in and i tend be overly defensive of my child. To the point of picking up a boy and taking him over to his mum and telling him to stay there unless he was going to play nice at playgroup cos she was too busy chatting to her friend (and is every week).
I however dont recommend doing this! As pp have said get your child to tell them to stop or tell you, if that doesn't work talk to the parent, only then would i talk to the staff at the play centre asking them to take action.