I have thought about the same thing - a market app that allows for the filtering of content for those of us that want to allow our children to get an android phone at some point in the future. I don't understand why google hasn't done that already? They have tags in gmail, how hard would it be to allow for tags in the market, and then the filtering of those tags within the phone's settings menu.
Hopefully someone will be able to come up with something... If I had more time I would see about putting something together...