It really wouldn't be.
Do that at OSI layer 3, every routed packet would have to have an "was this sent by a kid's device" flag.
Higher levels aren't the ISP's role. Lower levels, you'd need duplicate hardware specifically for kids, which couldn't connect to the adult's internet.
In the real world, we're still arguing about something that was already embarrassingly slow in its roll-out during my degree, and I graduated 20 years ago: IPv6.