There are really two parts to the ending of The Lion, the Witch, and the Wardrobe. The first is the unsurprising one: Good triumphs over Evil and everything works out for the best. Although we find this exciting, we're not shocked. The narrator's insistence in the extent of Aslan's power and the book's address to child readers both strongly suggest that the good guys are going to win this one.
It's the second part of the ending that we find more surprising: after becoming kings and queens, growing up, and ruling Narnia for many years, Peter, Susan, Edmund, and Lucy are suddenly transported back to England, where they become children again. We can just about accept that, when time passes in Narnia, it doesn't pass in "our" world where England is located, but how can we accept that the four children go through the entire process of puberty and maturation, only to have all its effects reversed as they go back to being kids again? It's especially hard to believe this when the children still retain memories of their time in Narnia. Could it really be possible to remember growing up and how you thought and felt when you were doing it, but at the same time to lose that maturity and go back to being a child? We don't really think so, but the book asks us to suspend our disbelief and go with it.
As an ending, we find this whole "and then they were kids again" thing unsatisfactory, right up there with the "it was all a dream" and "we went back in time and changed things so it never happened" endings. Here's the only excuse we can make: if you keep reading the other Chronicles of Narnia, you'll notice that C.S. Lewis isn't really all that interested in puberty, maturity, or adulthood. He's interested in the forces of Good and the innocent power of childhood. Whenever Lewis's child characters start to grow up or mature, they get tossed out of his books and he invents new children to be the new protagonists. So, in a fictional world where adulthood disqualifies you from participating, it actually makes sense that Lewis would keep turning his characters back into the children they were at the beginning. He wants us to see that their souls, not their bodies, grow up.