Any book that reaches people is a kind of children's book. It has its appeal to the imagination. It has to speak to the child within the adult.
For his own part, the writer must allow the same, have the child mindset enough to see the world not as an inanimate object, but a total living being inclusive of our own, therefore worthy of our highest respect, by which we see selfish behavior as a sin against the world.
This begins to account for Dostoevsky and his crowning accomplishments. He understood children, he took notes from their conversations. He presents everything clearly, as a child would, no bias.
A serious writer.