What might scare or upset a kid in fiction is totally, completely random. I was only terrified of two things as a kid- anything you told me was going to be scary and the bird from A Bugs Life
Yes you should not show some things to kids because its way more likely to, but discomfort is a thing you grow around and you can turn off a movie where something upsetting happens and process it, you can't do that irl and something upsetting will inevitably happen to 99% of kids.