You can call and report the abuse anonymously and here is a link on what to do and how to do it...
http://pediatrics.about.com/cs/child...ting_abuse.htm
(scroll down to your state and it has the info needed)
I would try to give specific dates/times and what exactly you witnessed so that they can see patterns and that might help in what they do. Maybe the mom/dad just need a wakeup call and will start taking being a parent seriously.