1. Especially with the popularity of the Harry Potter books and movies (although that fad may have waned), the role of magic in The Chronicles of Narnia is somewhat controversial. (Mind you, I haven’t read the Harry Potter books.) In addition, the role of magic in The Chronicles of Narnia intersects with the role of myth.
2. Consider stock characters from Greek mythology (e.g. satyrs, centaurs, dyads, naiads, Baucchus, Silenus). That’s a lazy creative shortcut. These creatures belong to a different fictional world history. They are products of Greek mythology. They belong to the fictional universe of our own world. It would be preferable for Lewis to invent characters that reflect the implied world history of Narnia.
Lewis might counter that in the world of the story, what’s mythical in our world may be real in another world. In principle, that’s a legitimate approach for a fantasy writer to take. Even so, there needs to be a consistent backstory to explain their existence in another world. They can’t just be abducted from our world, then stuck into another world. These creatures arise in a polytheistic context.
3. That said, this isn’t entirely ad hoc on Lewis’s part. He may well include these characters to illustrate his view that in divine providence, pagan mythology is a preparation for the Gospel.
Of course, that sympathetic outlook is at variance with how the Bible views pagan mythology, but my immediate point is that from Lewis’s perspective, this isn’t just a creative shortcut but a matter of principle–even if his principle is misguided. A considered judgment on his part.
The relation between (2) & (3) exposes a point of tension in Lewis. He sacrifices artistic consistency at this juncture to illustrate myth becoming fact.
4. In defense of Lewis, consider the character of Tumnus. When a satyr is the first character Lucy encounters, this shows Lucy, as well as the reader, not only that the wardrobe is a portal to another world, but a different kind of world. If what she discovered was an alternate Oxfordshire or parallel London, where everything belongs to the same kind of world, then that wouldn’t have the same effect. Making Lucy meet a satyr on first contact is an economical way for Lewis to show the reader that Narnia is truly out of this world. In a way, he makes the same point with talking animals, but the satyr adds an exotic visual touch.
5. In the world of Narnia, magic isn’t equivalent to witchcraft. What is magic in our world is natural in Narnia. It’s a different kind of world with different laws.
The White Witch is no exception, because she’s an intruder from another world. Her kind of magic wasn’t native to Narnia. Rather, witchcraft is like an invasive species.
And, of course, the world of Narnia has a Christian subtext lacking in the Harry Potter novels. So it’s not comparable in that respect.
6. In fairness to Lewis, we should judge the novels by the standards of children’s literature. They don’t have and can’t have the same literary sophistication as adult novels like Until We Have Faces or Perelandra.
In addition, Lewis was learning the ropes when he began the series. Honing his craft as a fiction writer. Moreover, he was breaking new ground in the genre. So they have a certain experimental charm that would be lost if he wrote them after gaining greater experience in the art and craft of fictional narration.