My point exactly.

Kids are going to use whatever their parents put in their hands. All of my nieces and nephews has some type of smart device (tablet, or phone). They are on it all the time. They never paid for it but it was given to them by their parents.

If parents stop giving their kids smart devices, then they will use that spare time doing something else (e.g. video games, homework, hanging out, smoking, idunno). There's no need for a study at all. Kids, in general, don't have money to buy an $800 device.