Good question. I guess we don't fully know if it's worse for kids because they've not grown up yet. There's a chance (non-trivial, I'd guess) that screen addiction in childhood could end up being harder to kick than adult-onset screen addiction. Also, even occasional use of drugs has been shown to be worse in children than adults. This could have the same disparate impact.
But have you noticed that people who read books sit very still, as if transfixed, with the only movement being rapid flicks of their eyeballs?