http://www.childhelp.org/
The worst thing you could do, is do nothing irregardless of 'pissing someone off' or 'worrying about hurting someones reputation.' The damage to the child for doing nothing is far worse than the above mentioned.
They usually investigate every report and if the father isn't doing anything wrong then he has nothing to worry about. My uncle was reported for abuse when his children were in elementary school, and he just had to go to an interview and they quickly concluded that he wasn't doing anything above what is accepted punishment in the US. Nothing goes on his record, they didn't do follow-ups or anything like that. He was never informed of who made the claim that he was abusing his children and to this day he does not know.
As mentioned by the other spudders, hard evidence would be nice but, you're not the police doing an investigation. You are just a concerned citizen who has insight into a child's life that others around that child may not see. I think it would be in your best interest to pursue legally reporting the suspicion to a group like the one I linked to above or another organization not only for the child's sake but, to help with your conscious as well.
You questioned whether or not someone could send the information for you, I don't recommend this. Only because the credibility of the claim suffers if all the other members have to go off of is what you say in your post. In all honesty, you could be making this up, so you're the only one that can make the claim because you are the one with the suspicion and you are the one that has seem the child. I know this isolates you and puts alot on your shoulders, but taking steps like you had mentioned of informing your parents and guidance counselors (I know they seem useless but, its things like this that they are there for) will hopefully assist you with the problem. If this all seems insurmountable, don't try to go at it alone, get an adult you can confide in to help you.
I apologize for the long post, but the importance of the issue necessitated that I speak up. I truly hope that nothing comes out of this and that it is just a misunderstanding but, if it is not you could be helping someone who needs it.