In general, The Lion, the Witch, and the Wardrobe is written in simple, plain language that many child readers can understand. (It makes a great "early chapter book" for developing readers.)
Yet its brilliant construction enables readers of all ages and backgrounds to enjoy the world that the book creates. You can think of C.S. Lewis as the Hemingway of children's literature—at least in terms of the simple, accessible prose with deeper meaning behind it and beautiful, stark construction.
Mood-wise, however, Lewis and Hemingway are miles apart—C.S. Lewis has characters binge on Turkish delight, and Hemingway prefers his characters to binge on booze and bullfighting.