I use hospital ID bracelets and write our children's names, our cell phone#s and the resort we're staying at on the wristband. I always keep extra in my bag because people always ask where we got them. (I work in a hospital but I'm sure doctor's offices have them too).
Our kids have never gotten separated from us at WDW, but unbelivably it did happen on our cruise this year. We lost our 6yo DD for 40 terrifying minutes at the Mickey pool during the Sailaway Party!! Luckily we had already signed her into the kids club so she had her bracelet on. A CM took her to the Club and they paged us right away, but with the loud music and panic setting in, I didn't hear the beeper going off for quite a while. Strangely enough, we had JUST gotten through telling our kids if they got separated from us they should STAY PUT and we would find them. I had her in my sight in the pool, then left the table I was sitting at to put my feet in the edge of the pool near her ... it was so crowded we must have just missed eachother... DD panicked when she saw the empty table and went to look for me - another parent saw her crying and took her to a CM - all while I was searching for her at the pool... very scary but I was glad we'd signed her into the club right away. Just didn't realize that was where the lost kids went...