I think you need to step in more. Next time DD runs out with a toy to hurl over the fence, go out there with her and tell the mean girls that they must return them at the end of the day. Even knowing you are watching might curb their nastiness. If course do it very nicely to set an example for DD.

If they ask her for food, go tell them to ask their mother...nicely.

Or just pop your head over and tell them not to ask DD for her toys anymore, and to be nicer because she is little.

It does suck a fair bit, you just have to keep a close eye on the little buggers.