You don't even need to take away their phones; in iOS there's this function called "Screen Time" where you can declare limits on kids' phones, like "no more tha

You don't even need to take away their phones; in iOS there's this function called "Screen Time" where you can declare limits on kids' phones, like "no more than two hours per day, and nothing at all after 21:00, except for Music.app". There's also analytics (how much time was spent over past week, with what apps) and a way for the kid to ask for more time. I'm sure Android provides similar functionality too.