Verizon has a Family locator as well, but I think it uses VZnavigator, so it wouldn't work on a Droid.
You could use MyTracks by Google, but of course your child could see it running and turn it off.
I've installed Mobile Defense, so if I loose my phone I can go to a website and see where my phone is. It doesn't show up in the installed apps, so you can't see that its installed. Of course that doesn't solve your tracking problem. You will just be able to see their current location.