A college student (hi, Meredith!) emailed me a few days ago to ask me about the "effective aspects" of a children's book. All good, except I had no idea what she was talking about. So, she clarified that she was wanting to know what a good children's book was. This is from my answer:
What makes a good children's book depends on the particular book in question.
A story picture book should have all the elements of story, engaging writing, a hero who grows and changes, and the best fit art for the protagonist and tale.
A concept book should convey the concept (be it, say, alphabet, numbers, colors) in a clear and engaging manner, one that will engage young minds.
If rhyme is used, it should be flawless and sophisticated.
Humorous books should be funny. Adventure books suspenseful and exciting. Mysteries intriguing. Fantasies imaginative. Gothics scary.
A children's novel must do all that an adult novel does, but the hero and sensibility is that of a younger person. They are generally a bit leaner, though, less self-indulgent on the part of the author. The audience tends to have a shorter attention span.
No kid reads a book because of what the New York Times has to say. To them, it must sing.
Basically, a good book should be the best book it can be, in whatever manifestation fits best for its unique nature. The same could be said of what makes a good person--one that lives up to its fullest potential and exceeds expectations.
As an aside, for the most part, literary children's books are written with a higher vocabulary than adult books, and for the most part, this is appropriate. What matters is the best word for the purpose, not its reader level.
But if the book is designed specifically for emerging or reluctant readers, the author will take that into account. Likewise, if the book is part of an easy reader line, the author's challenges include crafting a story that is so engaging we fail to notice the limits placed on the prose. It must transcend its form while staying within it.