This is something that seems so common sense - but according to the people at Disney - they had never seen it done before.

We took our kids (6 & 4) to Disney for the first time this past Christmas.  Crowds at this time of year are pretty thick, so we tried to come up with a way to "label" them in the event they got lost.

My husband went into a local drugstore and got a roll of clear packing tape and a bunch of plain white nametag stickers.  Each morning, we would fill out the tags with both of our cell phone numbers (Mom cell - xxx xxx xxxx / Dad cell - xxx xxx xxxx).  Then we stuck the tags on the back of their shirts with a piece the clear packing tape.  That way, they wouldn't play with them and pull them off.  (no names or anything).
Other parents as well as several Disney workers thought it was a really great idea.  And it gave us just that added extra sense that someone would be able to contact us if either of them got lost. 

We also attached these cute little child locater devices to each of their sneakers.   We got ours online at My Precious Kids.   It was a splendid trip - and no one got lost once.  :)