I wouldn't do anything about it.
Are they hurting you?
Although they are obviously in the wrong, if I were you I'd just be happy that I helped someone else out by designing them a logo. I don't see what the point would be of taking them to court, just because you can sue doesn't mean you have to.
If they ever make millions ask for your share then, otherwise just I'd let them be, its probably just some kids having fun (i haven't checked their site).