Yeah, well kids have tried that. The answer is basically, if you don't answer your phone in x minutes, you're grounded. And get a mobileme acct instead of this crap and double check they're where they say when they say it.
Mine would probably cook up an excuse like: "no coverage", "battery died" , "ringer was off for school and I forgot to turn it back on". Something like that. We'd argue back and forth and I'd probably lose. Kids have immense reserves of energy when it comes to that.
But anyway, I'm not sure I'd use this tracking service even if it was free. It destroys trust - and it's a poor substitute for real communication.