So, his parents don't care what he does, he deliberately hurts kids and seems to get a kick out of it. To me it sounds like he is either doing it all for his parents attention or he genuinely doesn't know how to play with other kids. He probably doesn't have much attention paid to him at home unless he does something wrong. The same goes with hurting kids, he probably doesn't know how to be a part of the group. If I were you I'd sit with your DD and when he comes near ask him to join in and teach him how you would like him to play with her.