You're focused on "password gating" for installing apps, but the largest subject here is websites. (also a nit: very few Linux systems are set up with noexec on home directories, I know "portable apps" exist for Windows, and I assume MacOS has similar dynamics)
> the absolute level of demand for parental control features has been low until recent years, and even this recent wave of demand is somewhat manufactured.
I don't agree with this. I think the demand has always been there and has been sort of discarded. I've personally done some of that discarding, in my younger days when the worry was of violent content but still on desktop/laptop computers where use was generally socially legible to parents. But these days we're dealing with pocket-sized devices that are no longer socially legible, plus malevolent commercial interests drawing kids in to get them hooked on dopamine drip loops.
But you seem to think the problem is solved, so tell me: what exactly are parents supposed to do here? I'm a new parent, we're still at the stage of watch videos with mama/dada, and play with the calculator app. The next step is probably curated sources of content/apps with general web browsing locked down (including self-curated things like perhaps a local copy of wikipedia). But then after that? What's the next level of expanding their scope look like, without them being subject to attack by corpos showing them ads/social media/weird slop shit/etc? Especially if they are going to have a SIM card such that I can't just filter most of this at the network level.
I haven't researched it all, and I'm sure there are some solutions. But I'm also more capable of seeking out bespoke solutions and actively choosing to use one, as opposed to the average person who wants things to "just work" and isn't going to delve too hard. Can't we agree that the pressure for this shitty legislation is coming from somewhere beyond merely Faceboot's money?