My kids have dog tags that we made at The Living Seas with their info on it. Only problem was my son kept taking his off so I threw it in the stroller pocket to avoid losing it. Of course he chose that moment to run off.
But here's the problem, and the problem that any ID tag won't really help: my son CHOSE to run off, therefore he did not consider himself lost at all. We had gotten ice cream from that shop in between the Pooh ride/Snow White store/Dumbo ride. We sat down at a table right next to Dumbo to eat the ice cream. My son ate about half, said he was done, and proceeded to get up from the table and sit in our stroller, which was right next to me (though facing out). My daughter then sat next to him in the stroller. I cleaned up the table, threw out my trash, and my friend (who was sitting with us, with her 2 kids also) asked me where my son was. I was sitting right next to him and had NO idea he was gone. Neither did my daughter, who is usually very aware of what her little brother is doing.
We searched all over and nothing. I then went to a CM working at the Pooh gift shop outdoor cashier, told her what happened, and she stared at my blankly and said she could not help!!!!! (In her defense, she had a Japanese flag on her tag, so maybe she didn't understand me, but she should have gotten help or been trained in what to do, and when I told Guest Services their response was 'oh well, sorry!")
Anyway, knowing my son fairly well, despite my true panic and terror, I went to the Pooh playground and there he was running around.
So to me, he was lost. I was so scared and panicked. But my 3yo son was totally happy and at ease and not 'lost' at all. In fact, as I was approaching, he was leaving and heading back to the stroller.
But what it taught me was that dog tag or not, clearly sometimes nothing will help as your child has to realize he's alone and be unhappy about it, which mine was not.