I think you've made some really good points here but I also think that there's a limit.
I agree, I would have started with reading the book myself or finding out in more detail about what DD was struggling with in particular and having a conversation.
However, I don't think we should just assume that children should be able to deal with any kind of book, regardless of the theme. We don't all have the same tolerances for everything and some books could be too challenging - especially if it's a trigger point for you personally.
Since I can remember, I have been interested and enjoy reading books that include murder, death, and medical themes. I've been fascinated by them, truth be told. But I really struggle with books that have strong themes of loss, grief and bereavement, and I hate horror/paranormal themes.
I love books and I love reading. I always have. But I have a visceral reaction to what I read. I can remember reading a biography of a man who died from AIDS in the 80s, and I put down the book and quite literally sobbed my heart out.
So I think yes, books are useful to teach our children about the world, about history, and for fiction, to provide insight and to support emotional development. But I don't think that means that every child should have to endure reading something that they find personally really difficult. For example, a child who has been SA might have a very different threshold for someone who hasn't, likewise a child who's lost a parent recently/traumatically, etc.
I just don't really agree with the notion that books are "just words" and that we should all be able to endure every book, even if it is age-appropriate. Some books are just really difficult, for a variety of reasons, and I think it's unhelpful to insist that words are meaningless.
(Might be worth mentioning that I'm autistic/ADHD and I tend to experience really big second-hand emotions so that might explain why I get overwhelmed when reading sometimes!)