Just get a SafeDriver Wireless Vehicle Monitor. Plug it in, set up your PIN (1234 is not allowed) and it is ready to go.
The device will record the highest speed, the distance that is traveled and any instances of sudden braking.
It is available from Thinkgeek for $69.99.
A bargain considering the great Big Brother power that goes with the device. And that's the point. Do you really want to keep track on your kids like this. Or is there still some old fashioned trust left somewhere?