"mechanism to enforce that is parental control on devices."

Meh, I use it, but it's super annoying and I think that with my Daughter I'll take a different approach (but it will be some years before that is relevant).

On Android: The kid can easily go on Snapchat (after approval of install of course, and then you can just see their "friends") before Pokemon Go (just a pain to get working, it keeps presenting some borked version which led to a lot of confusion at first). I just lied about his age in a bunch of places at some point. Snapchat is horrible and sick from our experiences in the first week.

On Windows: It's a curated set of websites (and no FireFox) or access to everything. It's not even workable for just school. Granting kids access to our own minercraft servers: My god, I felt dirty about what the other parents had to go through to enable that.