Heh, I was bout to suggest "wouldn't it be awesome if such a thing as a temporary tattoo with your info exists?"
The problem with things that can be attached to clothing is, if the worst case scenario happens and someone tries to take off with your child, it's something that an intelligent criminal might recognize and take off first thing. Something attached to the shoelaces? Take off the shoes or cut off the laces, give the kid something to make him or her sleepy, then walk out of the park singing a lullaby as if it were your own child fell asleep in your arms...
Even if a tattoo with your pertinent info can't be done, consider a long-lasting tattoo of ANYTHING, as long as a: You remember what it is, and b: it's something that can be found on a child's body without any invasion. For example, a spider web tattoo on the elbow. Something a lot of kids would think it cool to have on them for a week or two, and only requires a child to roll up his or her sleeve if someone is trying to abscond with them. Your child goes missing, you tell security first and foremost "he's got a spider-web tattoo on his right elbow." It's one thing to have ink with names and addresses on it, it's another to just have a symbol, one that a kidnapper might not think twice about. And if you really want to get high-concept/low-tech with your security, next to the tattoo, you write his name or your name or something important...but in invisible ink, the kind that only shows up under a UV or other specialized light. This way, if someone does try to take off with the child, but is stopped by security, and the kidnapper insists "This is my child," it would be a quick way to double-check the kid's identity while waiting for you to arrive from the other side of the park or wherever you might be.
