There is a simple and better way to do this, which is device-wide age status attestation. That is, the whole device or user account has a 'minor' flag set, and passes it on to software, and so on.

Governments are not pushing for this because this is not about protecting children, it is about removing privacy and increasing control.

This only addresses one axis of your concern, but if they are accessing YouTube via desktop browser (or Firefox on Android!), the "Youtube-shorts block" extension gets rid of the Shorts UI. You can still watch Shorts, it will just display them in the normal video UI without infinite scrolling. It's a huge quality of life boost.

Although obviously this does nothing for those using the mobile or TV apps.

Even though it's a bit easy to disable, you can use the "Unhook" extension to turn off Shorts.