Adults are much harder to reach because they can always simply go buy a new computer or phone and circumvent any measures put in place to help them.
On internet use restriction the best solution so far has been to use a simple electronic plug timer connected to a laptop without a battery. Zap - no internet for you.
There ought to be a solution that installs itself into the computer's firmware though. Most of us on HN install new operating systems at a much higher rate than the typical user so having a persistent solution would be huge.