Mothers: what would you do if some other child hit your child?

Not with their hand but with a rock! It just happened. My two year old was playing with his dump truck in the park when another little boy of about three or four came up. They started playin together which was fine. But then he decided to take my sons truck away from him. When my son wouldn't let go the little heathen hit him in the head with a rock and tried to run off. By that point I had gotten there and gathered my son and the truck to go inside. The little boy started yelling at me inspanish. I have no idea what unit he lives in or if his mother speaks english (unlikely) but the whole area is overrun with little naked mexican kids who don't share their toys and steal from others. This isn't the first time one of them has stolen something from my son, they have taken his cars and his bike from him but never has he been injured by on of them. He has a huge goose egg now. I'm so mad. I'm tempted to just never take him to the apartment park again even though the next closest is several blocks away. I'm all for sharing but my son is entitled to play with HIS toys without being harassed, right?