For starters, forbid behaviour and engagement surveillance. Without that data it would be impossible to fine tune the apps to be addictive for each individual. Just with that, it'd probably tackle most of the "device addiction". Then if you want to go the extra mile, consider forbidding things like personalised recommendations, infinite scrolling, auto-play-next, loots and even notifications on by default.

Might as well just make session and cookie variables illegal, since persisting any kind of state may be able to be misused.

Easy. Just make it illegal (with massive fines) to advertise to and track kids. Kill the business model and solve the problem.

It's such an easy sell too. Hey parents, any objections if we prevent companies advertising to your kids? How about stopping them from tracking everything your kids do?