Spying on the kids

Is there a cheap, perhaps even freeware spy program that lets you monitor what goes on, on the kid's computer? That is if I ever get it working, just got the Router and 100' either cord...

...but only one computer at a time can go on line for some reason.
I'm running a PC and the kid has a Mac, can that have anything to do with it?
Anyway she's only 13 and I'm not sure I want her surfing the net without a leash. Can we monitor her computer from ours, if we ever get this router working right?

Thanks