You can't be with your child 100% of the time. They are spending significant time with others, e.g. in school. Those people you can't control.

Doomscrolling or porn is just too "appealing" to children, like sugar. Children don't have their minds fully developed to be able to say "no" to them.

If in school everybody has a smartphone and does doomscrolling, your children will do as well. Or they'll be ostracised.

A hangout for 11-16 year olds often seems to devolve into a bunch of kids all watching their own phones. It's really depressing to watch, though they do seem to play as well.

We have had several arguments about no social media and we're only 1 out of 6-ish years in to the too naïve to look after yourself on the internet phase, and the eldest already figured out how to download some chat app I'd never even heard of without permission.