The Australian government has a webpage that is all about how to deal with a child or young adult disclosing abuse to you. Now, obviously the rules and requirements may not be the same where you are, and the list of who to contact is specific to their Australian audience, but I think a lot of the information about child abuse victims is pretty applicable to anyone, anywhere.
Check it out, and if you live in Australia, definitely keep those resources close at hand if you deal with children on any regular basis!