I'm with Lex. If I knew someone (child or adult) was being abused or neglected, I wouldn't hesitate to call the police. A child may have no one to help him or her. With an adult, s/he may be too afraid to go to the police, so I'd do what I can; if I tried to help him/her a few times and s/he refused to leave the guy/woman, then I'd leave that situation be. But a child has to depend on adults to help.

I'm going to say this, even if others don't like it: If you know of an abusive situation, don't know if anyone's doing anything about it, and you don't try to help, you're guilty, too. You're either part of the solution or part of the problem.