@Ruth_Mottram how many hours per day do they spend with a book? Make it a 1 to 1 between books and phone and the calculus changes.
The phone is a most miraculous device. There is no point to starve a young brain from this incredible window (*). The main problem imho is it is making extinct another most miraculous device: the book. Me thinks a brain is hungry enough to feed on two power foods. But a balanced diet is needed.
(*) of course _what_ you do on the phone plays a big role too...