I've used 2 things. One is a "dogtag" from
www.usdogtags.com with their name, our names, and our cell numbers on them. When we travel, I print a label with our resort and the resort's phone and attach that to the back. These are worn UNDER their clothing (to avoid getting caught on something and also to avoid prying eyes). I grew up in the Army on a foreign base, so dogtags were part of our daily dressing routine, only now they come in fun colors!
The other thing we have is silicone bracelets (like the livestrong ones) from
www.rekeydesigns.com. Those simply have "Mom: {cell#} Dad: {cell#}" They wear them all the time (I order backups in case they break) and I like knowing that our numbers are handy to them in case of an emergency.
The other thing I recommend is as soon as you get there, start pointing out cast member name tags and instruct the kids that if they get lost, they need to find one of those tags *behind a counter or loading a ride* at the closest place where YOU (the parent) got lost. Point out that there are people who wear name tags, but the don't look like the CM ones.
The biggest thing I've done to keep my kids from getting seperated from me is to have them hold my beltloop. Anytime a crowd gathered or they were starting to lolligag, they hook a finger in my beltloop and they can look around all they like without my getting lost. 3 of my 4 didn't particularly like to hold hands, so this was a perfect solution.