While I was still in my single digits, I read a wide range of books, including systematic theology, eschatology, lots of sci fantasy, and whatever the editors of Reader's Digest Condensed Books selected. I think that, for the most part, the content which I was not emotionally mature enough to absorb just went over my head.
I tend to think that children's understanding of age-inappropriate themes and images in books (however you define age-inappropriate) is partially limited by their real-life experience of those themes, and their sensory experience of the images. If your experience does not include a related traumatic or explicit situation, the words you read are more academic, rather than resonating intimately with the actual emotional experience that you have had.
