I agree with controls on addictive/exploitative platforms like Facebook or Instagram. These can be feasibly controlled at the source.
In principle I agree with keeping some content away from children, but I don't think any of the implementations will work without causing worse problems, so I disagree with implementing those.
in the day to day practical way
There's a world of difference between practically required and it being illegal to use anything else, even if initially for a small set of population. You still have a choice to avoid those now. Moreover there is a fairly large subculture of gamers etc opposed to these movements, and open computing platforms will take a long time to fizzle out without intervention.
If you mandate locked down devices for kids, it will very quickly become locked down devices for everyone except for "licensed developers", because no one gets a bunch of new computers upon becoming an adult, and a new campaign from big tech will try to associate open computers with criminals.