[astrobe_]

Here some types of small motorize vehicles can be used by persons under 16. Of course, they have built-in speed limiters.

What if we had this kind of distinction for mobile devices with Internet access? Adult's smartphone vs kid's smartphone?

It could be some sort of combo of a specific "for kids" SIM card (with, say, DNS blocking at ISP level and/or date of birth of the user) and a "kids hardened" OS that prevent bypasses and the parental monitoring tools you mention. The OS (in a wide sense, that could extend to the browser and other mission-critical programs) could be certified by some national agency; commercial products have to pay for the test, FLOSS projects could tested for free on request.