Actually "completely disconnected"? That would be impossibly expensive.

Easier but still hard would be e.g. a child-mode[0] for consumer[1] operating systems that can only connect to a DNS resolver which itself only resolves certified-by-age-rating content. This would "only" be "extremely expensive": people complain about the cost of getting a film an age rating, and they're only 1-3 hours long and don't get near-continuous updates.

My guess, and it is a guess, is that content for children shouldn't be "the internet", but a small and specifically curated collection for each age range[2], essentially the back catalogue of all the things we've already made for kids, from Saturday morning cartoons to Scholastic books. Of course, this is still flawed, hence my parents' generation fretting over D&D, Harry Potter, and anything about sex education (and twice over if it was gay).

[0] or a multitude, corresponding to film content rating systems: https://en.wikipedia.org/wiki/Motion_picture_content_rating_...

[1] Despite having learned to read with the Commodore 64 user manual, I'd say that if you can get into a command-line OS today with all our easier distractions, then you shouldn't even get asked how old you are.

[2] Even single shows split content in this way, e.g. Sesame Street has different muppets be representative of different developmental stages of kids.