That is a risk with all fantasy.
The trouble is that, even in Narnia, it presents a mythology that can be viewed as an alternative to Christianity. I have known a few people who dabbled in the occult because of their love of the wizards in LOTR.
But I have also known a few who found it a great bullwark to their Christian faith.
A few people will becomes obsessed with any form of fiction that has a huge fan base, especially films. Just look at (for instance) Star Wars, Star Trek, Harry Potter, and the Twilight franchises. In all of these cases, there are a small number of people who take their involvement with the films beyond mere fandom into obsession.
I don't think that's a reason for any rational person to avoid enjoying any of them.
(mind you, there is a different reason to avoid Twilight...it's terrible)