The fact is that no 8-year-old is going to read and understand the morally ambiguous and complex world of pain, necessity, betrayal, and sacrifice that us middle-aged women live in. It is actually not possible for a book written for an 8-year-old to take place in an adult world, with adult concerns and adult struggles.
I agree that obviously a child's world has different concern's to an adult's - you don't get many children's books about mid-life-crises, the problems of retirement or how hard it is being a middle-aged male academic who wants to sleep with his hot students (though I consider the last a point in its favour.)
But children don't live in a separate world to adults. They live in this world. They suffer as much trauma, bereavement, loss and fear as adults do - you only have to hang out on Mumsnet for an afternoon to read about abuse, complex moral battles in which the children are central, bastard fathers who don't want to see their kids any more and mothers posting things like 'I don't like my child'. They're actually more likely to suffer abuse and neglect and live in poverty than adults are. And you don't need to study much child psychology to know that eight-year-olds are capable of highly complicated and contradictory emotions.
An eight-year-old may not understand all the intricacies of Anna Karenina. But as all of you who've been reading the Elena Ferrante novels can vouch for - that doesn't mean their own morally ambiguous and complex world of pain, necessity, betrayal, and sacrifice is any less complicated or interesting.
It can't help but be woefully simplified and black/white.
Ooh, that's fighting talk. And untrue. Take the last book I read on this thread - The Thursday Kidnapping. It's not a perfect book - it goes a bit loopy at the end, it's rather snobbish, there's a car crash that comes out of nowhere and it's very 1960s. But it features child mortality, Hungarian refugees, child kidnapping, class issues, an incredibly well-observed portrayal of a teenager living a life of severe emotional neglect - and possible abuse - with very accurate attachment issues, a car crash which is not only the fault of our heroes, but has at least one serious casualty, possibly fatal, PTSD and stress, questions of responsibility and fault, and moral problems such as what moral responsibility do you have towards someone you feel desperately sorry for, but do not actually like?
And yes, it could also be read by an intelligent eight-year-old - though I think it would sit in the 9-12 section of a bookshop.
No, it's not as intricate as Anna Karenina. But its villain is a hell of a lot more psychologically complicated than any I've ever read in an Agatha Christie.