AT&T now offers a $10/month plan that gets you the realtime location of two of your plan's phones ($15/month for five). But unlike other family trackers, this one doesn't appear to need parental permission
One of our editors tried it out, and was able to track members of his family without them knowing (UPDATE: Text messages were sent to the tracked phones, but notably not a single person who received them understood what they meant and ignored them)—and without his knowing the master password to the AT&T account. You just create a separate login for the service and voila. AT&T says in that case, the master account will be notified—by SMS, email and snail mail, with information on how to revoke the other person's permission to track numbers.
When Boy Genius tried it, they tracked phones totally silently, that is, without the phones receiving any indication of being observed. AT&T's policy states they'll be sent a notification once a month. Also, BGR mentioned that the service was able to access GPS position data on phones that had GPS—it wasn't just triangulating the phones using cell towers.