1. The current norm of social siloing apps was created by these tech companies in the first place. What regulators can do is discourage anti-competitive practices that lock users into specific software and hardware platforms. If there's plenty of competition for every kind of social app, and competition for OSes, and users could freely choose and move between them, then not having a particular app would not result in social isolation. This affects adults as well.

2. The OS has a firewall. But it's currently not user-controllable on your phone. Phone companies have decided you don't need that feature. But actually, they can easily implement a nice UI in the settings for the firewall and lock it behind a password, then parents would be able to use it to block individual websites. We can even make it possible to import/export site lists as a txt file so that you can download/share a curated block list that you or other parents made, to block many things at once. You could also do this for your entire home WiFi network in your WiFi router's settings, if your router's firmware has that feature.

And yeah, I agree that we should make the platforms less evil in general. But I think the way to do that is to give people the ability to easily ditch bad platforms and build new ones. Let the platforms actually compete, then the best will prevail. Right now, they don't prevail because of layers and layers of anti-competitive barriers. It would take great technical effort to regulate all the tricks these tech companies use, that's why I propose dealing with it at the root: make it so that all computer/phone hardware manufacturers must open-source their device drivers and firmware, and let the user lock/unlock the bootloader and install alternative OSes. If we do this, then the entire software ecosystem will fix itself over time along with all the downstream problems.