Are we sure parental control methods were proven to be fundamentally inadequate for the situation? There's no bulletproof security method to guard this data, so the discussion is about weighing privacy loss vs child safety, against existing methods (or improving other methods). Also the choice to set the age is in the hands of the parent, so I don't see the benefit besides enabling the kid to choose app in a more self served manner (which you probably don't want to allow).

Seatbelts are there because it's obvious you're not in control of other drivers, even if your car has all the safety controls. The downsides are minuscule in comparison to the privacy discussion, in my opinion.