www.safetytat.com
They sell medical grade tattoos that come printed with your phone number or any other pertinent information that you have printed on them (i.e. peanut allergy, or autistic) for this exact purpose.
We have used them, and I have to say, they do provide a small sense of comfort, just knowing that the information is there, if he would get lost in a crowd. And I'll take extras for all the kids in our group, so at least if one is lost, someone will call MY cell phone.
Also like some others said, I taught my son very early on that if he ever got separated from me, to find a mommy - and I'd make him tell me how he'd know if someone was a mommy. He'd say "it's a lady with kids." I've told him to find a mommy, and show them his 'tattoo' and have them call me. As a mother, I know without a doubt, that if a lost child came to me and asked me to call his mother, I would not let that child out of my sight until he was safely returned to his mom, and I'm sure that's true with 98% of mothers.
Women, mother's especially, are far less likely to perpetrate on children than any other population (I believe). And telling kids to find a security person, or a staff person is too difficult for small children... uniforms look different at different places, etc., so they don't always know what they are looking for, or they are hard to distinguish. Mommies are easy to find.
I also like the idea of the i.c.e. bracelets, except that they could come off easily at a place like an amusement park or something. Still a good idea though. Might make some. :)