May I ask how old they are?
Im 15, and I would hate it if my parents tried to do such a thing..
But thats really none of my business.
Anyways....... Im not sure exactly what your talking about but (assuming you have a credit card) get yourself a nice keylogger.
Install it on their computer when they are away, and let it do the work for you. Basicly what a key logger does is what it says, records keystrokes. And it will send you logs and dates via email. It sends you ANYTHING typed, including passwords, backspaced words etc..
Ive heard good things about that one ^^
Check out that site and browse around. But the thing is they pretty much all work the same, so I wouldnt worry too much on what you get.