The way I expect this to go down is that Android/iOS/etc. will roll out an age bracket API call in the near future and tie that API call to some set of foundational permissions (e.g. internet access / file access / etc) - they have done this in the past, notably for API deprecation.
A minimally invasive implementation of this will likely only restrict apps running if the OS itself is being run in a kids-mode .
(But that isn't what the laws actually require)