Loopt works like that, in fact you can have it update your location in facebook too. Google Latitude does this also. The only thing that may or may not be different is that with both loopt and latitiude you can turn it off in settings. Not sure about the sprint one. If you can trust your kids though to leave it on, either of the free options would save you $5 a month.
By the way, Latitude is built into Maps. It's one of the settings.
Edit:
You could also see if wavesecure would work too. In that program if you send a text to your phone it will send the location of the phone to whatever phone or email you have setup. There is even an addon to wavesecure that makes it so that if someone tries to uninstall it the phones locks and they can't use it.