I think in order for parents to do this, they must themselves use their phones as tools and not entertainment.

Kids are great at spotting hypocrisy, and if parents are seen zombie-scrolling through Instagram for hours, and then they tell their kids not to zombie-scroll because it's bad for you, well the kids will know their parents are totally full of shit.

It's impossible for parents who are themselves addicted to smartphones to tell their kids not to be addicted to smartphones. The parents need to cure their own addiction first.