According to generally accepted figures, one in four people in the UK will experience at least one episode of mental illness. So a lot more than that will have at least one loved one who does.
It's estimated that one in three marriages in the UK ends in divorce.
Around 20% of families are headed by a lone parent.
At any given time there are around 65,000 children in care, of whom only around 40% will return to their birth family.
There are also around 100,000 adopted children under 18 (OK, not many of those have been abandoned, but many of them will have a degree of curiosity about their birth families).
5980 children and young people 19 and under die every year. Most of these have siblings and/or friends, so the number of children affected by the death of a child is far higher.
It's easy to dismiss this as "the negative stuff in life", but it's reality for a lot of children in the UK. And even if it's not reality for your child, it may well be reality for one of her friends. It will almost certainly be reality for one of her classmates. And books that acknowledge that, and have stories about people in those situations learning, growing, healing, moving on are IMO pretty darn positive.
Let's face it, Anne of Green Gables, recommended on this thread, is a story about a girl whose parents died when she was very young, who has spent years languishing in the care system and who, having finally been adopted deals with the death of her adopted father while she's still quite young.
Charlotte Sometimes, recommended on this thread, features the death of one of the protagonists.
And they are bloody good books, that are read by generations of girls. Even if they do give a taste of the negative stuff in life.