We just laminated the information with a hole in it (actually, we used one of those cheap sticky luggage tags) and pinned it to their clothes. Small enough that strangers couldn't see it. When the kids wore pockets, we pinned them inside the pocket. Otherwise, we tried to pin them someplace discreet.
But we ended up losing one tag, and leaving them in the room. My son (5) was concerned without his tag that if he got lost, we wouldn't find him. So we took the autograph pen and wrote dad's cell phone number on his hand. That actually worked great for the 4 and 5 year old - the are old enough to be able to know their name and that the number on their hand is Dad's phone number. (It wasn't hot and we didn't get wet - don't know if this would smear in humid weather or if riding Kali). The nice thing is that this involves no planning and no props - other than finding a pen and carrying your cell phone.
We also introduce our kids each time to "Mickey's Friends" and show them the cast member badges (the white ones, the red ones are guest of honor badges). If they do get seperated, they are supposed to find one of Mickey's friends and tell them.
So far, we haven't lost one - at least not at Disney.