Like the other Narnia chronicles, The Voyage of the Dawn Treader is written in a straightforward, easy-to-understand, conversational style – as long as you're thinking of having that conversation with a slightly stuffy but thoroughly good-hearted professor. The sentences tend to be short and the words are chosen precisely but simply, with nothing wasted or elevated. You can almost imagine that C.S. Lewis himself is telling you the story over dinner (we picture him smoking a pipe for some reason). The word choices and styles of phrase are straightforward and accessible, the kind of thing you can easily imagine someone saying to you, rather than being dense or more stereotypically "literary."
For example, when Lucy notices the shadow of the ship as it moves across underwater hills, the narrator speaks to the reader and makes an everyday comparison to clarify what's going on:
It was like what you saw from a train on a bright sunny day. You saw the black shadow of your own coach running along the fields at the same pace as the train. Then you went into a cutting; and immediately the same shadow flicked close up to you and got big, racing along the grass of the cutting-bank. Then you came out of the cutting and – flick! – once more the black shadow had gone back to its normal size and was running along the fields. (15.4)
Notice that there are 64 words in this passage, and 48 of them (75%) are just one syllable. Of the remaining 16 words, all are just two syllables, except for the word "immediately," which is four syllables but also a relatively low-level vocabulary word. You probably didn't need to go looking it up in a dictionary. We don't usually get all statistical when looking at literature, but in this case we think the numbers are pretty telling.
If you're not into syllable counting, then consider the distribution of the parts of speech: there is only one adverb, "immediately," and the adjectives tend to be very simple: "bright," "sunny," "black," "big." Clearly, Lewis is choosing to write in a more basic prose style in order to reach the widest possible audience.