My mom used to worry about that when I was a child, so we did the entire "password" thingy. We had a password (well, it was actually a phrase, a disney phrase heh) established between myself and my parents, and if the situation ever came up where someone other than a trusted relative needed to pick me up, I was supposed to ask them what the password was. Every once in a while she would quiz me to make sure I still knew, and she made sure I knew not to go telling anyone randomly about it. Luckily, the situation never came up though. But I plan to do this system when I have my own child.
Wrighter had some great suggestions. Perhaps you could get your children one of those lanyards with an ID holder and put a card in there that has their name, your hotel, and your cell phone number in case they get lost. And to easily identify them, maybe have them were a certain bright colored shirt or hat?