Sportmanship?

Jack has been asking me to take him to a football club for a few weeks now, he is very passionate about the game. One of his friends from school plays for Seaton Carew under 9's, so tonight I took him there. His friend wasn't there, but that didn't put Jack off, he just got straight ino it. They were only training for an hour, so I sat in the car (it was raining for a change) and read a magazine in between watching him. When training was over, he came back to the car, I asked if he'd enjoyed it to which he replied he had, but the other kids had been calling him names all night and saying that he was rubbish.
I told him not to worry and that he wasn't rubbish, and to ignore them. He still wants to go training again on Saturday morning, his friend will be with him hopefully.
I've come home absolutely livid with those kids, I can't believe that kids that young have already got such a nasty side to them or am I just being naive?