Think about EU standardizing USB-C for charging. Many people were up in arms: Do you want the state to decide these things? However, personally I benefited. Now I have to carry around only one charger for all my devices.
If the govt standardizes non-smart phones for kids we will all benefit.
Of course this doesn’t address tablets, netbooks etc.
So the actual answer is good parenting, which I posit is one of the bigger problems in the US today and has been for a long time.
Or we keep pretending that pushing education and passing laws about cell phones will somehow be a substitute for bad parenting. The US has been doing that for a long time as well, isn’t working.