The police at the Royal Easter Show are encouraging parents to write their mobile phone number on the kids’ arms. Apparently they still get the 15 to 20 lost children a day but simply ring mum and dad and they are quickly reunited. How simple and effective and bloody obvious once it is mentioned.
Our local amusement park sells buttons (badges) with a space to write the parents’ cell phone numbers, for a pittance. I’ve seen kids wearing them at other large events, so the parents obviously keep them after visiting the park. (They have the park logo printed on them, too - never pass up an advertising opportunity!)
Because of all the sickos out there, I would caution writing a cell phone anywhere that’s visable to the general public. When the HallKids were little, I always wrote identifying information (their name, my name, our phone number) on their tummies. It was covered by their shirt, but easily accessed if they were seeking assistance from a police officer, or, god forbid, unconscious.
Brilliant idea. Filed for future reference in trupa’s noggin.
We do this with the kids at Disney World. Sharpie on the hand. They are old enough to say “this is Dad’s cell phone.”
It’s not compleatly new, college kids haveing been doing it at least since I was in college. Just wasn’t mom’s phone number, usually a friends, or their address so someone could point them in the right direction after a party.
Doesn’t this just make you want to cry thinking about it? 
What a world.
I once sent my college roommates out on an acid-enhanced tour of Boston with their names and our phone number written on their hands. When they finally came home, I was informed that they desperately wanted to call me for a pickup at some point, but couldn’t find a payphone. 
I got lost a lot as a kid, and I can’t imagine how much easier it would have been for everyone if I’d had my mom’s cell number written on my forehead. If cell phones had existed at the time, of course.