If you haven't checked these out before, you definitely should give them a try. Boys and girls alike will love them, particularly age four through seven.
But Julie Andrews also writes novels. Our two favorites are Mandy and The Last of the Really Great Whangdoodles. These books are in the sweet spirit of A Little Princess and The Secret Garden, so if you have a little girl who loves those stories (even just the movie versions), give her Mandy to try. And if you have a boy who loved The Lion, the Witch, and the Wardrobe, and they like fantasies like that, give them, The Last of the Really Great Whangdoodles.