Generally, I'm the last person to suggest we use technology to control people's lives and what they view. However, it's also the duty of parents to help teach their children how and when to use the media. Most commonly today you see things like the V-chip, which let parents block their unskilled children from seeing shows with certain "ratings."
A far more useful concept, I think, would be a device which limits the amount of time children can spend watching the TV. What they watch in that context can be mostly up to them, and if they understand the concept of a time budget, it will probably improve not just how much they watch but what they watch.
A PVR like the Tivo, or in particular, the DirecTivo, is the ideal platform for doing this. Children would get their own remotes, or a code to enter on the master remote to start using their weekly budget of TV hours. Once the budget was used up, they could not watch TV for a while. With a PVR, this would not block them from seeing a highly desired show, but it would delay it.
If two children wanted to watch the same show they could both enter their code to halve the amount of TV credit used, encouraging sharing and (minimal) socialization. Siblings would pretty quickly develop a market, trading TV hours like prison cigarettes with one another for real-world things, even money. This need not be discouraged. Random TV surfing would be discouraged, and commercial viewing strongly discouraged.
Adults would have to take the burden of having to enter their own code for unmetered viewing, a price they would pay to cut down their kid's TV hours.
Of course there are also some privacy considerations to consider.Parents could modify the formula a bit. For example, they could grant hours of entertainment in exchange for chores or good marks. They could also budget different classes of prgramming, allowing unlimited news, documentaries and educational programming with limits on cartoons and stupid shows.
Certain temptations must be resisted. One should not log what people watch, no matter how much parents might want to spy at this level. There are of course already tools to do that but one can limit hours without restricting content. Ideally a child limited to a budget of TV will automatically start to keep their watching to the very best stuff (by their definition of course.)
Naturally some children would move to watching lots at friends' homes, as they already do to bypass existing TV restrictions parents place.
Such a system could be bypassed on many TVs by a smart kid. They could bypass the PVR unless it is tied with the satellite receiver or cable box. Once they get old enough to figure that out it may be past the time to control their TV watching.