"No book is really worth reading at the age of ten which is not equally (and often far more) worth reading at the age of fifty." If C.S. Lewis was right about this, then a good test of the quality of a given "children's" book should be whether or not adults can (not whether they do) enjoy it as well. To put it another way, if it is only a children's book, it is probably not a good children's book.
He's right, of course. Consider those books that are called children's classics. Peter Rabbit is considered a classic. So is Winnie the Pooh. So are many fairy tales, and so also (though for different reasons) are the Little House books. Children love these stories -- but the same is true of the adults who read them to the children. Something in them goes deeply enough into a person to obviate the question of age. A child may be delighted by a story in different ways than the adult who is reading the same story, but it would be surprising if those elements of poetry and romance (yes, in Beatrix Potter!) that delight the adult did not also delight the child, not because of some remnant of the child in the adult, but rather because of the human in both.
On the other hand, there is a class of books written specifically for young people which is nearly impossible for adults to enjoy. Nor should it be said that we shouldn't try to enjoy them because they are written for young people. That would be a great mistake. These are the teen series of the pulp or school book club variety wherein some teenager "learns about life" through an adventure (in boy's books) or through a relationship (in girl's books). In these books, most of the elements that make children's books so delightful are lost. The supernatural, the world of faerie, talking animals -- all are gone. Some might respond, "and good riddance, too! Escapism is all right for children, but young people need to learn about the real world." This response shows how badly literary fantasy and the purpose of stories in general is misunderstood -- and what assumptions lurk behind such a remark about "reality"?
Something else is gone, too. In the best books, children are taken seriously as people -- young, yes, but people nonetheless. In the other kind, they are talked down to in the attempt to give them "their own literature." If they have their own, and we have ours, how will they make the transition? How do children's minds become adult minds? What is the essential difference between the best children's books and the best adult books? It is not one of kind.