This is my opinion as a person who speand a shït load of time on my phone:
In my opinion, the total time spent on platforms have a negative mental effect on kids because of:
-Firstly, the content they are watching and the things the producer promotes (For example, if the producer promotes spending less time on their phone, they'll think about doing it)
-Secondly, the media being toxic and bad. (If you express your opinion, and if its different than the stereotype opinion, then the media will call you different things and will aboid talking to you based on how you think.)
Then again, the amount of time spent on it could be for a greater good, learning new things, that could help you in life.
Most of the time its useless things, or things that are concerning to parents, things that dont promote good language, or good behavior. These things can change the kids' mental behavior, and physical, by a lot.
In conclusion, the subsequent negative mental effect on kids can be prevented, though its very hard, and it only depends on the content they see.
Dãmn, hope i helped u!!