Hmmmm. I personally have a problem with trying to stop my DS from pushing and retaliating, so I work really hard at getting him to come tell an adult if he's been pushed / bullied. Of course, the mischevious me would point out the other mother and tell your son to go tell her that her son hurt him!

I think though that "hold on, hold on - we play nice at play centres" is acceptable to say to the other kid, and if it continues then tell the staff that there is a kid who is nasty etc.

and good luck.