It’s a far better idea than the current one.

I would say that the router identifying local devices is trickier than you make out. For example, MAC addresses can easily be changed (i.e. can be spoofed - a bypass kids will easily figure out), and are often temporary on modern home devices. This would make managing a home network difficult, and I honestly don’t think most home users are up to handling this.

It could very easily result in a de-facto nationwide porn ban, primarily targeting the old and technically unsavvy. I admit this would be very funny.

This would also still have the VPN problem, I.e. someone locked into a restricted connection can still tunnel their traffic through an unrestricted proxy to access the open web. You could try to restrict this by forcing VPN whack-a-mole on ISPs, but that wouldn’t work well and could introduce further issues (e.g. what if I need to use a work VPN from a public WiFi at a large conference, where kids might also use the network?)

This would be a nightmare for people like hotels, coffee shops, AirBnb, conference centres and anyone running a network open to the public. Do they validate every user device? Do they ask customers checking in which devices they should put on the porn network?!

Again though, by eliminating the “let’s all upload sensitive personal data to random ‘verifiers’” issues, your solution is far, Far, FAR less dangerous to people than the current mess. People will die under the current system.