Fact: parents and children alike will read and enjoy Prince Caspian. Some might chalk it up to good storytelling, and, yeah, a lot has to do with its classic fantasy tale of good vs. evil and all that. But the writing style might have even more to do with it. Check this out:
There was a pause so long that the boys began to wonder if Nikabrik were ever going to begin; when he did, it was in a lower voice, as if he himself did not much like what he was saying. (33)
The first thing that will probably grab your attention is the word choice; it's simple and straightforward. With the exception of Nikabrik's dwarfian name, there's not a single word in there that will give any reader—tall or small—much hesitation. Very kid-friendly.
The second aspect worth noting is the sentence structure. Adults often wonder what it is about children's books that put them to sleep, and the answer is generally not the word choice or subject matter so much as the sentence structure. Simple sentence after simple sentence after simple sentence, and the adult reader… starts… to get …zzzzz.
So Lewis employs many variants of sentence structures in Prince Caspian from simple to compound to complex to even the grand compound-complex—as we see in the quote above. He also mixes and matches them without leaning too heavily on one or the other.
The result may not be readily apparent to you while you read the story, but your brain will be thankful for the variety—whether you're an adult or a child.