Allowing someone to serve who has abused or molested a child in the past. If a person has ever abused or molested a child, they should not serve in Children's Ministry...period. Yes, God forgives and perhaps they can serve in another ministry in the church...but not in Children's Ministry.
Being disloyal to your Senior Pastor or Senior Leadership. If you can't follow the vision and direction of the church, then leave quietly.
Immorality. Put personal boundaries in place for you and your staff. If you are married, one boundary would be to never ride alone in a car with a member of the opposite sex.
Allowing a volunteer to be alone with a child. Never allow anyone to be alone with a child. There must be two volunteers present at all times.
Not having a check-in and check-out security system in place. This is a non-negotiable. If you don't have computer check-in, then use peel and stick name tags with matching numbers.
Not running background checks on everyone who wants to serve. Do they cost money? Yes. But you can't afford NOT to run background checks.
Not reporting abuse. You are a mandatory reporter of abuse. Have steps you take before reporting, but when it's to the point where you need to report...then report!