The most common fear in 6 year olds, if I remember my child psych classes from way back when, is death.
A little girl named Michelle in my class got hit by a car and killed when I was in Kindergarten/1st grade, and apparently I was inconsolable for a long time over that whenever I’d think about it.
I think it’s the age when we can really begin to grasp the depths of it and what it really means.
Kids are older than we think they are. They do a lot of pretending-to-be-happy for our sake. Of course they’re also happy too, but kids don’t like to see adults upset… until they’re pre-teens.. then they kind of enjoy it.