If I were going to ID a kid, I'd probably write my cell phone number and the cell # of the person I was with on the edge of the top of their foot (where the laces aren't rubbing all day) and tell them that if I got lost to find a staff person or a mom with kids and ask her to call that number. I'd go over what Disney People are.
And I'd go over and over it with the kid. What do you do if I get lost? "Find a Disney person or a mom with kids." What do you tell them? "My mom got lost. She said that if she got lost I should show you my foot." What do you show them? "My foot." What do you do when you're worried about me? "Find a Disney person or a mom with kids." What do you tell them? "My mom got lost. She said that if she got lost I should show you my foot." What do you check? "My foot."
And I'd always, always phrase it as ME getting lost, not them. Easier for a kid to think you're lost than they are. That way, they aren't freaking out, "Oh GOD! I'm lost! I'm scared! What do I do?! Oh, noooo!" They're more like, "Oh, no. Mom got lost. She said she might get lost. Now what do I do? Oh, jeez. I have to show someone my foot."
I'd write my parent's or in-law's number on the other foot so that they could talk to the kid and reassure them that they were okay...and talk to the Disney people in the event that our cell's weren't working to help locate us.
That way, it is out of sight. Can't get lost. Won't rub off.