Get business cards printed with your name, home phone number, cell phone number, and work number on them. Any time you leave your child with someone, hand them all the information they’ll need in case they have to contact you.
You can order free + shipping business cards from http://www.vistaprint.com the first time. When you travel with your children, whether far from home or on day trip, place one of your “mom cards” in their pocket in case you get separated. Peace of mind can go a long way!