I met a little boy that used a backpack one when I was a CM. For whatever reason the backpack tether seemed more humane then the harness leash type ones. They just have a single clip on the front and a regular backpack on the back and a section to clip the tether. You can unclip it if on a ride or the kid is behaving. The little boy went on and on about how his special monkey backpack kept him from being a bad boy and with his best friend he is always good. It was very cute.
It was this one
http://www.skiphop.com/zoo-little-kid-safety-harness?174=21&gclid=CIv4uNHVodICFVtXDQodnJkHqw I have seen them at Barnes and Noble in the kids section. I see a lot of little kids form my nieces preschool have them as their "back packs" and then only use the tether part if need be.