I think that's the center of the problem. In the current times online media and mobile apps are designed to maximize the participant's screen time by being as addictive as possible.
Even grown-ups face problems self-regulating with regards to an "adversary" that invests huge resources in getting our eye-balls (and data and ad views).
Restricting screen-time becomes more and more difficult the more kids grow up, I do not think that this is a very scalable solution.
I think it's more important that kids are made aware and learn how to deal with this "adversary". Make them aware how the TV (or smartphone) "catches" them when they actually had planned to do other things. Make it a game to find out, together with your kid, how a device can have this kind of control over a person. Kids are curious, they will want to know about mechanisms and incentitives of the system and I think this can be explained even with the vocabulary of a 6 year-old.
Humans are very adaptable, maybe kids growing up in our current environment will just acquire skills that the older generation does not have and has difficulties to imagine.