From Michael Moorcock’s essay “Epic Pooh,” as archived at Revolution SF (it was originally written in 1978, revised in 1989, and revised again to note recent authors):
The sort of prose most often identified with "high" fantasy is the prose of the nursery-room. It is a lullaby; it is meant to soothe and console. It is mouth-music. It is frequently enjoyed not for its tensions but for its lack of tensions. It coddles; it makes friends with you; it tells you comforting lies. It is soft:Wait a minute! Milne’s characterization of Eeyore is hilarious. But his narrative voice is indeed the voice of an adult looking back/down on childhood. Nesbit’s narrative voice, on the other hand, is an author in conspiracy with young readers.
One day when the sun had come back over the forest, bringing with it the scent of May, and all the streams of the Forest were tinkling happily to find themselves their own pretty shape again, and the little pools lay dreaming of the life they had seen and the big things they had done, and in the warmth and quiet of the Forest the cuckoo was trying over his voice carefully and listening to see if he liked it, and wood-pigeons were complaining gently to themselves in their lazy comfortable way that it was the other fellow's fault, but it didn't matter very much; on such a day as this Christopher Robin whistled in a special way he had, and Owl came flying out of the Hundred Acre Wood to see what was wanted.It is the predominant tone of The Lord of the Rings and Watership Down and it is the main reason why these books, like many similar ones in the past, are successful. It is the tone of many forgotten British and American bestsellers, well-remembered children's books, like The Wind in the Willows, you often hear it in regional fiction addressed to a local audience, or, in a more sophisticated form, James Barrie (Dear Brutus, Mary Rose and, of course, Peter Pan).
Unlike the tone of E. Nesbit (Five Children and It etc.), Richmal Crompton (the 'William' books) Terry Pratchett or the redoubtable J.K.Rowling, it is sentimental, slightly distanced, often wistful, a trifle retrospective; it contains little wit and much whimsy.
Perhaps the “voice of the nursery-room” is one reason why some of the novels Moorcock mentions—Lord of the Rings and Watership Down among them—are treated as books for young people even though their authors conceived them as books for adults.