When we took our then five-year-old granddaughter to Disney World for her first time, we picked a very friendly Cast Member when we first entered the park (it's easy to do in the morning) and showed Marlie what the Cast Member name tags looked like. We told her that if we ever lost her, to find someone like that and tell them we were lost (kids never get lost, adults lose them). The Cast Member we picked agreed with us and told Marlie that any Cast Member would be able to help her find us. That's really all you need to do. Have your kids go to a Cast Member and let them handle it. Disney World has a great system in place for lost parents. Make sure all kids in your party have ALL cell phone numbers. Don't just give them one as you might have the phone turned off or not be able to hear it. Marlie had three cell phone numbers on her. She also had her name, her mother's name, and her resort name (NOT the room number). Remember that little kids will sometimes get really flustered when you have lost them so it's not a bad idea to have their names printed somewhere.
Also, if you have a digital camera, take a picture of each child before you leave in the morning. It'll be easy to show that to a Cast Member instead of trying to remember what your child(ren) wore that day.