Could you make a bracelet out of it? I do think in the event a child is lost, that the cast members would look all over the outside of the child for identification.
You may also want to make sure you have a current picture of your DS with you when you enter the park. If you have a digital camera, take a picture at the start of each day of him in the clothes he will be wearing that day. This will help tremendously if he should get lost.