Now that you know about the abuse you must inform your local child protective services office. The safety of this child comes first. You can keep confidential all of the other information the child shared with you. If you are not allowed to provide treatment to an underage client, suggest that the girl ask her parents for consent for treatment or offer to meet with them with her to ask for consent. If your call to protective services has irreparably damaged your relationship with this girl, help her with a referral to another counselor in your community.
There are many professional codes of ethics offered by different professional counseling groups for their members/certificants. You may want to check the American Counseling Association, and the National Board for Certified Counselors websites, for answers to questions about counselor ethics.