Disneyland is
great with lost kids- every cast member (employee) in the park is tasked with identifying and helping them... oh, here's an article which details the procedure:
http://www.mouseplanet.com/akrock/akrock7.htm
We lost my oldest DD once, when she was around 5... or, well, it turned out we had, we hadn't noticed! I was with my parents, and we had split up for a few minutes and gotten confused about who had which child, and nobody had DD1. And the parade was arriving into the area we were in, so it was quite crowded. So I'm done with whatever I was doing, and was probably doing a scan looking for whoever had DD1 and saw... a CM, at the curb, as the parade was going by, talking to DD1. Wha'?? Apparently DD1 had scooted to the front of the crowd to watch the parade, and then realized that nobody was with her and looked confused, which the CM spotted immediately- they were in the process of exchanging names and starting to scan for DD1's adults (me and my parents) when I spotted them and we were reunited. DD1 never even really knew she was lost, and was never scared.
Disneyland rocks. Seriously.
Anyway, all that said, I think the animal-backpack leashes look the least annoying, and always make sure you know who's got what kid, with positive handover. "You've got DS"+"I've got DS." Writing contact information on them is also a really great idea.