I may well being oversensitive here but here goes,
It sounds as if you are trying to protect your child (nothing wrong in that) but from what?
Its more than protecting her from 'sad' things, its almost like protecting her from children whose lives are somehow 'sad'.
Like my little boy (told you I was sensitive). Surely if you shield your little girl from the subject you will have to shield her from the children who are living the circumstances which you describe?
What is that going to teach her about others? That children living without both parents are to be avoided? What about children who have lost a parent or sibling or children in foster care or have been adopted?
All these things are likely to upset your DD as she sounds a sensitive little soul but I dont feel you will help her deal with this by pretending these things dont happen.
My little boy is the same age as you DD. He has had to deal with losing his birth parents, being adopted, coming to terms with his early neglect, losing his adored adopted sister.
He is a lovely, happy (if somewhat complicated) little boy. Not someone to be avoided or to protect you DD from.
I know you are talking about a book in this instance but there are real children behind those storylines.
This is NOT meant to be a mean post. I am genuinely confused about what you hope to achieve.
YANBU for wanting your DD's life to be nice and fluffy and to protect her BUT I cannot agree with what you want to do it.
Talk to her, show her MORE books not less, show her that even kids that have suffered multiple tradgedies and traumas can be happy and healthy.
I think that will stand her in much better stead than hiding RL from her.