This is billed as Gaiman's first adult book in nearly a decade, which is not, I think, strictly true. It's not a children's book because it doesn't seem possible to write it as a children's book, except it's written in a way that is, more than anything else, evocative of Diana Wynne Jones's books for children. Partially, this is because Gaiman writes his seven year protagonist as a child and not as an adult's idea of a child. He writes like a children's author, at least insofar as he writers a character that is meant to be relatable to children. The boy at the heart of this novel feels like a child's idea of a child, not an adult's. This is a good thing.
Additionally, I had this sense while reading the novel that...everything was going to be okay. Or okay enough. That the bad things that were happening were the bad things that happen in fairy tales and that, to paraphrase G K Chesterton, can be defeated. I spent the novel trusting Gaiman as an author in a way that I don't usually trust authors when they ostensibly write for adults.
Still, content wise, this was not a book that adults would really feel comfortable reading to children. (The fact that it felt more like Coraline than any of his other books says more about the author than the book, I think). It was a deeply immersive and almost painfully compelling story, one that taps into the part of our psyche that feeds off of fairy tales and memories of childhood. It feels true, even if not a factual representation of a childhood that happened. And it was beautiful in the way that stark prose can be beautiful.
If you've ever read Diana Wynne Jones's Time of the Ghost, then you will be familiar with this genre where the author mines his or her own childhood to create a sense of the fantastic and a feel for the things we may or may not dare to believe as children. If you haven't, I highly recommend that as well.