I just saw a TV ad for Wherify Wireless. They sell a GPS wristwatch for kids along with a monitoring service that lets you keep tabs on them. Supposedly it can also tell if the device has been removed. It also has a pager and can call 911 - the latter can be disabled.
They are developing a locator for cars too. This has been around a while for major corporate clients, but now it would be available for the rest of us.