It depends on what you're looking to do. Latitude (part of Google Maps) does a pretty good job by itself. There are countless "send your phone a text message and get its location" apps, but I don't know if the text message alerts. I'm experimenting with setting up an SSH tunnel with reverse port forwarding from my own phone to my server so I can log in remotely - but that needs to be manually started at this point.
Obviously consider the ethics of what you're trying to do as well, tracking a 12 year old and letting him know it's so mommy and daddy don't lose him is very different from planting a spy app on your 18 year old's phone without his knowledge