Perhaps I'm being snobbish - but I suspect the snob is Faber. Anything a famous writer for adults deigns to write for children must be a classic, ipso facto. We should crown them by acclamation.
Faber have form in this area. It's always bothered me that the Collected Poems of T. S. Eliot disdains to include what are probably now his best-known poems - those from Old Possum's Book of Practical Cats. Being written for children, they are beneath notice. I remember I wrote to Faber to complain about this some thirty years ago: I'm still awaiting their reply. (I will add, though, that the more recent Complete Poems and Plays does include them.)
Sometimes, a poet known as a writer for adults may be shown to best advantage in their work for children. For example, perhaps the best thing Gertrude Stein ever wrote was The World is Round (1939). (It is also a far more enjoyable book to my mind than the outwardly-similar The Little Prince, which it predates by four years.) Here are the opening lines, to give you a feel:
Once upon a time the world was round and you could go on it around and around.
Everywhere there was somewhere and everywhere there were men women children dogs cows wild pigs little rabbits cats lizards and animals. That is the way it was. And everybody dogs cats sheep rabbits and lizards and children all wanted to tell everybody all about it and they wanted to tell all about themselves.
And then there was Rose.
Rose was her name and would she have been rose if her name had not been Rose. She used to think and then she used to think again.
Suddenly, instead of being irritating, Stein is revealed as a really good children's writer. Every picture is enhanced by the right frame - and for writers, the frame is genre.
Is The World is Round included in The Collected Works of Gertrude Stein, you ask? No, it is not - but perhaps only because, as far as I know, no such book exists.