Most parents assume that professionals who work with children pass background checks. Unfortunately, background checks only pick-up convicted sex offenders or those proven to be abusers by child protective services. Abusers who move from state to state may be missed. Religious institutions, sports leagues, and other private organizations often don't require background checks, and no one monitors private instructors.
Here are two general resources that I particularly like from the National Center for Missing & Exploited Children to help parents protect their children from sexual abuse and exploitation.
Both publications are also available in Spanish. Hard copies can be ordered at 10 cents a piece with the first 50 copies free.