I have used several different child IDs for my DD5 and DS3.
The last time we were at WDW, we used tags on the shoes that actually get fed through the strap. We purchased them at kidtagz.com. They never slipped off their shoes and the tag fit so tightly and was just hard enough to snap that the kids couldn't take them off even if they tried. We wrote our contact info with a Sharpie.
I'm not sure if the shoe tags will work for us this year because our kids like
Crocs. We will probably be bringing ID cards that we have purchased from kidzidz.com. The kids wear the lanyards and we just drop it down inside their shirt. By the way, if you've never seen the cards you should at least look at them. They are great! Pricey at first, but you have unlimited renewals for updated pictures, phone number change, etc. until the child turns 21 for free (you do pay for shipping & handling though).
Another thing we have used is a bead bracelet that we purchased from lorisnursingjewelry.com/gpage2.html. More girl styles than boys, but my DS3 has one as well with appropriate boy beads. I use these sparingly though because the kids like to play with them and I'm afraid the kids will take them off.
Slightly off topic, but the other thing we did was take a picture each morning so we knew what our kids were wearing. That way we had an accurate description in case they did get lost. We also started our day in the park showing our DD (who was then 3) how to find a cast member and to recognize the uniform in the event she did get lost. We told her to look for the name badge and then we would stop and speak to a cast member so they could point it out to her. We would have her practice saying to the cast member "I can't find mommy or daddy. Their phone number is on my shoe."
Hope some of this helps. Good luck!