So, I have a runner. Well, she was a runner when she was younger. Now, she isn't, but she's also 13 now. For our first trip, she was a relatively oblivious, wandering child with ADHD... so, she would see something and walk away without ever realizing it. More than once she walked away with a stranger at places because she saw something, went after it, then saw someone about my build, and would grab their hand. So, my point is that I've been there with a runner. I've lost her at the local science center (thankfully only in the preschool area, so the doors are too heavy for the kids to open). I've lost her multiple times at the park when I've stopped to talk to someone.
So, heading to
Disneyland with a wandering runner, I investigated and considered all kinds of things--sharpies on arms, removable tattoos, dog tags on shoes, alarms that go off when she was more than x feet away, GPS trackers, and so many other things! In the end, I went with a dog tag ($4.95 in
Walmart, in a "make you own dog tag" machine at the front of the store). I put my name and phone number on it. When we got to the park, the first thing I did was point out the cast members and what their name tags look like, and reminded her many times that if she can't see me, she needs to find a person with a name tag, and show them the tag on her show. I believe someone on here said they were told by a CM that they can't move clothing or touch a child when looking for contact information, so you have to make sure that your child knows to lift up their sleeve/pant leg/back of the shirt/where ever to show the CM.
In our personal experience, it wasn't needed; but, it was also 1-on-1, so my attention was usually on her anyway. We're taking our oldest grandson next summer, when he will be a newly-7-year-old, and he's a bit of a "free spirit" as well, so I think we will probably use the dog tag on the shoe for him just to ease my mind.
In the case of littler kids, I'd totally recommend doing a leash--either a backpack one (my daughter loved her's when she was a toddler/preschooler, as she was proud she was carrying her own sippy cup in her "purse"), or a wrist one (it has one of those curly cords between the parent and child velcro bracelets). Not the best if you're fashion-conscious, but better than nothing!