In my post 2 days ago I wrote about W. French Anderson's conviction and sentencing for child abuse. How did he gain access to his victim? In addition to being a world-renown research scientist, he held a black belt in tae kwon do and taught/coached martial arts. His victim was one of his martial arts students. Since the California charges were brought to trial, a young man in Maryland (Anderson worked at the National Institutes of Health for many years) has come forward with similar charges.
Parents often put their children into martial arts programs to develop their self-discipline and character. Be careful with whom you leave you children. Pay attention to any teacher or coach who suggests "private time" with your children, especially if the don't want you around. Pay even closer attention to your child's response to that teacher. Martial arts does attract some interesting characters (I teach karate as well as self-defense, so I've met many "characters"); take some more time to look past the teacher's reputation and charm. Anything that makes you uneasy is worth investigating.