We have used Who's Shoe Child ID on each of our last trips. My son wore them on his sneakers and my daughter wore it on her sandals without a problem. We just left them on their shoes and didn't have to remember to put something on them each day. I have kept them on my son's shoes since we have returned.
We use these and they are awesome! We never needed them at Disney but my eldest wandered off at the Bronx Zoo one day. The security guard saw his shoe tag and was about to call when my DH found him!
My kids know that if they get lost, they tell a CM that mommy and daddy's phone numbers are on their shoes.
Amanda