There's been interesting debates in parlement preceding this measure, with several interesting position papers on the topic from researchers, and even student associations [2]. The researchers emphasize that adolescents are much more susceptible to the bad effects of smartphones, due to inexperience with dopamine and its effects on dopamine production, being easier to condition, FOMO. The main adverse effect they name is what they call a 'crumbling brain', with a short attention span unable to focus on one thing for a longer time. An often-repeated soundbyte is that students using smartphones often score in average 1-1.5 points less on tests, on a scale of 1 to 10.
I dunno what to think about it. As noted by the student association, it seems like children won't get the chance to learn how to handle the traps smartphones pose. Then again, I was free to use mine in high school and I'm still addicted to the thing :/
[1] https://nos.nl/artikel/2481424-kabinet-geeft-dringend-advies...
[2] https://www.tweedekamer.nl/debat_en_vergadering/commissiever...