Although a lot of these answers are technically correct I don't think they are based in reality.
Because of this:
Would you still say something if you thought that person could hurt you?
It's true, there needs to be more information in this posting. What is your relationship to the abuser? How can they hurt you? How deeply? Can that hurting change your life?
Having been an abused child myself, of course my answer would be yes. Report the bastard.
The problem is this:
1. Abusers are liars and can (and probably will) turn the accusations against you and it becomes a 'he said/she said' situation.
2. I have reported (several times). I even reported myself when I was being accused of being abusive, begged them to come to the house and see what was going on. Nothing. They did nothing. That made me a HUGE target in the house and within the extended family.
3. As far as it being anonymous. lol. Not.
4. Abusers do NOT take responsibility for their abusive actions. Ever. So the ensuing spin out will possibly - no - probably trigger off fits of violence, rage, acting out against the child, yourself, or anyone else that may be 'handy'.
I did eventually handle the situation by going to a women's outreach centre. I told my story and that made 'them' responsible for reporting. That is maybe your safest way of divulging the information. Be careful to make the information generic. If there is a situation where only you were a witness, then it will be obvious to the abuser that you are the one who told.
I suffered PTSD, was stalked, harassed for 2 years, terrorized, and my life was literally destroyed because I reported. It would be much more helpful if the system that is meant to protect children actually did. In my experience, it was a ton of grief for me and the children were never kept safe.