If you have children, read this thread

[voodooman]

Originally Posted by Rolo

They have been testing http://www.kidspotter.com/ at some theme parks (ex. at Lego Land in Europe). Dunno if its similar (they use WI-FI), but if I had kids, then I would put GPS transmitters on them, and follow them like it was 1984 ;-))

Rolo,
I did a massive amount of research on this before starting and found only one company has anything similar and thats the watch which is in my opinion, just to big for a child.
Kidspotter is a similar device, and as mentioned it uses WI-FI but does not have the tamper proof band.