We are going in 2 weeks with our 3 and 5 year old boys. I got name tag labels at
Walmart to put on the inside of their shirts. I wrote their name, my husband's cell phone # and my cell phone # and the Resort we are staying in. Hopefully we won't need to use this but just in case. I also plan to take a picture of them each day so I have a current picture of what they are wearing on hand if needed.
Please do not put their name on their shirt (even on the inside) unless it is written small. If a predator would see it, he/she could use the knowledge of your sons' names to win their confidence ("Hey, Timmy. Your mom said you should wait over here.").
Here is what we do: We print off index card sized sheets that simply say "If Lost" in big letters on one side. On the other side, we type again in big (but a little smaller letters) "If Lost:" followed below in small letters (unable to read unless you have it pulled up to your face) by our names, cell phone numbers and resort (not room number!!!!!!!!!). We then laminate the card, poke two holes in it, thread a shoestring through it and place it around our children's necks. Our youngest child is too young to know to show someone a cell phone number on their wrist, belly, etc. The only way she will wear the card is if her brother and sister wear the cards. We also put some Mickey Mouse stickers on the card before laminating it to make the card officially Mickey Cool.
Example:
IF LOST
Parents: John and Pocahontas Smith
John's cell phone: 555-1607
Pocahontas' cell phone: 555-1608
Resort: Animal Kingdom Lodge
Remember, print everything from "Parents" to "Lodge" in letters so small no one can read them unless they have the card close to their face.
We also teach our children (1) to stay where there are if they are not able to find us, (2) how to identify a cast member and (3) tell a cast member you are lost if you see a cast member close.
I suggest reading the Unoffiocial Guide section on children. The Unofficial Guide gives you some warnings about situations and places in the park where you have the greatest risk of losing your child.
Finally, we use a leash on all of our children four years old or younger.

Many people do not like the idea of a leash, but it kept our children from getting lost in a crowd of people.
As you can see, we are a little paranoid.
