I dont see why phones can't come with a browser that does this. Parents could curate a whitelist like people curate playlists, and share it, and the browser would honor that.

Combined with some blacklisted apps (e.g., all other browsers), this would be a passable opt-in solution. I'm sure there's either a subscription or a small incentive for someone to build this that hopefully isn't "Scam children".

It's not like kids are using PCs, and if they use someone else's phone, that's at least a severely limiting factor.

They do, don’t they? Apple devices have had a robust whitelisting/blacklisting feature for at least a couple of years. I use it to block websites and apps to lessen my phone addiction. I’m sure Android offers similar features