My favorite books from my childhood are those in C.S. Lewis's Narnia series. In particular, I love The Lion, the Witch, and the Wardrobe. Naturally, when I was a little mite, I didn't pick up on the Christian symbolism. Now that I am a Big Fat Atheist, I notice it all right, but don't give a hoot. Actually, that's not quite true: if anything, I rather enjoy it. I suppose it's because the books were already special to me.
More of a problem for me is the fact the Susan doesn't get to come back to Narnia at the end of the series. Why not? This is especially hard for a child reader to understand. A child reader wants the whole family together: the thought of an excluded one is hard to accept.
I can tell, even from the very first book that she appears in (T.L.t.W.a.t.W.), that Susan is not Lewis's favored child. She doesn't want to give in to the magical world. In Prince Caspian she is the last to see Aslan and is responsible for leading the children astray while they are on their way to join the battle. In The Last Battle, Susan does not appear and is described by one of the children who returns as "...no longer a friend of Narnia." But why? Is it because she is too cautious and sensible to believe (to have faith?) or is it because (as some critics think) she is becoming a sexual being, more concerned with her own beauty?
And what of Aslan's words in T.L.t.W.a.t.W.: "Once a king or queen of Narnia, always a king or queen of Narnia"? How are we to reconcile these things? Perhaps Susan just has to wait for her turn. Maybe she has a thing or two to learn first.
Neil Gaiman (most awesome modern fantasy writer) wrote a short story a few years ago called "The Problem of Susan." (It appears in his recent collection, Fragile Things.) This story features an elderly Professor Hastings, who is Susan Pevensie in her dotage. Worth a read. Gives quite a different perspective. Don't read aloud to children.