As others have mentioned, It's very important for the child to remember to stop and stay right where they are, as soon as they realize they are separated from you.
If they go running around looking for you, most likely, it will take much longer to reunite. If they stop and wait, you can re-trace your steps and find them quickly.
I've seen kids with a cellphone number written on the back of their shirt, near the neck. You can't miss it.
We did the dogtags from Walmart. We had no info on the tag, except for the 3 cell numbers, belonging to the 3 adults in our group.