Quoted:
I'm looking for something for an elementary school aged child. Seems a bit young for a cell phone.
I was going to suggest this as well.
It also gives you the added bonus of them being able to call mom/dad/911.
It also gives you the added bonus of being able to call them.
Google Latitude isn't 100% perfect however. But it works reasonably well. It works on android phones, and I believe they have apps for other phones as well.
No matter what you go with I would imagine you will be paying data fees in order to upload their location to a server somewhere.