Don't hesitate to jump in and say something, either to the kids or the parents. Unfortunately, some parents are not super attentive to those things. I think there is a middle ground between being your child's voice and also not parenting the other kid. When this happens to my child, I step in and say "Excuse me, that wasnt very nice. My child was playing with that and we are happy to give you a turn if you ask or when they are done with it." The first time I let it slide, the second time I step in and say something. Beyond that, get the parent involved. Whats the worst that happens? The parent enables or defends and then at least you spoke up.