I tell everyones kids off basically, but I do it a certain way.

Some kids seem to make a bee line for little ones to push over and I am more than happy to kneel down, put my hand on their shoulder or arm (no I DON'T care about touching other peoples children!) and say firmly - don't push her over, its not nice and look them in the eye until they break contact.
THEN I say, hey lets all go on the slide, and get them all having turns nicely and it seems to work every time.