So.. stop producing smartphones / tablets? TVs? What's your other option?

Stop marketing to them to kids or normalizing kids using them?

This is really not that hard. Modern app culture is highly addictive and provably poison for our brains over time. Adults can choose poison if they wish but when they give it to their kids it is called child abuse.

I'm not saying go to zero. But definitely not over 3 hours and definitely not a dedicated device for them.