"I would like a control box of some sort that controls the cable signal that comes into the TV. I want to be able to control the total time a particular child has in his account to watch, plus the actual channels that he is allowed to watch (no Playboy or MTV), PLUS the time of day that he can watch (not during home work time). Programmable by channel, cumulative time, hour of the day, and day of the week. I also need a master pass word for parental programming of the kids accounts, plus be able to watch what I want to at any time. The kids could use either an individual account number or an individual "card" of some kind with a PIN like our ATM cards. This "box" should be secure so that a 14 year old boy can't bypass it very easily. At least without doing come major damage so that I'd know it when he did it. I know that this is a lot to ask, but I'm very familiar with computer programming and chipset technology, I do know that such a thing can be done. I just don't have the electronics knowledge to do it."
The following probably won't help you build such a gizmo but here are some thoughts:
First, I would not attempt to build any of the RF/cable switching stuff - there are too many variations. I would suggest trying to control the control of what you have. With a cable box, this would be relatively easy - just put the box and an IR transmitter in the same sealed enclosure. If you have only a cable ready TV, you could substitute or intercept the remote detector signal inside the set and disable the front panel controls.
Then you need:
Each account would have a means of adding to the balance, password authentication, etc.
You would have a superuser account for your own watching as well as changing any of the individual account settings.
Too bad I don't still teach my intro to computer design courses - this would make a nice term project.
If you have a junker PC, this would be a simple bit of programming (but quite wasteful of power even for an 8088 based PC).