If the suspected abuse is taking place within a family, or between a child and another person who is in a caretaking role to the child, such as a teacher or day care provider, you should file a report with Child Protective Services.
If you suspect sexual abuse between a child and someone outside of the family who is not in a caretaking role, contact the local police department or law enforcement authority closest to the location where the abuse occurred.
You also have the option of filing with both authorities. If you file with an authority which is not best suited to take the report, ask them specifically who you should contact to file. Typically reports should be filed the area where you believe the abuse took place, not necessarily where the people involved are right now.
In many states reports can be filed with child protection authorities anonymously which means you can file without providing identifying information about who you are. If you have questions about filing you can call a confidential helpline such as Child Help USA or the Stop It Now! Helpline (1.888.PREVENT) to discuss your concerns before making your report.