Most TVs and some stereos have a "sleep" function which will turn off the music at a set time. But when you're putting a child to sleep you need something more gradual. A slowly declining volume, with a variable end-time.
I have a stereo with clock-alarm capacity, and one of its settings is to get progressivly louder at thirty second intervals (by the end of two minutes, it's quite unpleasant to be in a small room with it). Something like that, but backwards, slower, and adjustable.
It'll sell like hotcakes for the parental set. Within two years of introduction it'll be a standard feature on mid-range boom boxes. And, of course, the cute-kiddy stereo market is still underserved. Perhaps because good features didn't exist?