I don't know about iOS, but on Android I have access to Family Link which means I can control what apps my kids can run off the app store, and I can control whether they can access explicit websites (according to google) or have free access. I know other parents that are well aware of this tool, but they have to make sure those phones or tablets are signed into with accounts they have ownership of. I think this is the direction that the government should be pushing for and making sure apple google and microsoft are all playing nicely to allow parents to manage devices under the family.

iOS is similar. You can also limit apps/books/movies/etc. by content age rating, block adult sites, etc. without parental approval (which just happens over messages).

There is even an on-device image classifier for images/video to blur pornography from messages and keep them from sending it to others.