As David Leonhardt discusses in his New York Times editorial “The Conspiracy of Inaction on Sexual Abuse and Harassment”, and as I have seen repeatedly as an expert in institutional child sexual abuse cases, people often suspect or know when there is something wrong. Sexual abuse and harassment continue when they do nothing. The law does not require proof or evidence. The threshold for reporting is when someone suspects or has reason to believe that a child is being abused or neglected.
In situations where the potential perpetrator is known to the adult, this can cause enormous discomfort. But children who are unable to speak up or advocate themselves need to come first. If you see something that makes you uncomfortable, that makes you think abuse might be occurring, call your local child protective services hotline or 1-800-4AChild and talk about your concerns. The professional on the other end can determine whether or not further investigation will take place.