I find it doubtful that C. S. Lewis ever intended it to be convincing. He wanted it to function as a fantasy story, but I think he also wanted it to resonate with people who buy into the Christian myth, and to make people who don't think "Huh, so this thrilling fantasy story is something that all ties into Christian belief? I'd be interested in learning more about that then."
When I realized what it was supposed to be an allegory for, though, I was hit by a wave of plot elements I'd always accepted as perfectly sensible fantasy elements which suddenly struck me as crashingly wrong. It was like Hirou's revelation in The Sword of Good. Like when, just a few chapters earlier (I was rereading Prince Caspian, at the part where Aslan tells Peter and Susan that they won't be able to return to Narnia and would have to come to know him in their own world, and it finally hit me that he was supposed to be Jesus,) Lucy had asked Aslan why he couldn't simply raise up an army and crush Miraz like he did with the White Witch, and Aslan said something like "Child, don't you know such things are never done the same way twice?" I wanted to shout at him, "Why!? Justify yourself to me, Aslan!" The bottom dropped out of the plot and everything started to seem arbitrary and unnecessary; Aslan has the power to solve every problem, he has absolutely no need for the protagonists, has no comprehensible reason for selecting them out of the entire world population to fill their spots in the song and dance he's orchestrated, and the entire structure of the plot is determined by some set of supernatural laws or requirements which are completely opaque to the reader.
C. S. Lewis wrote that an allegory should be able to stand on its own merits as a narrative, but once I turned a more critical eye to it, I was overwhelmed by the degree to which the Chronicles of Narnia failed to do so. The grand structure of the plot rests entirely on authorial fiat, rather than any comprehensible chain of cause and effect. The offensiveness of some of the story elements (such as the Calormenes almost all being unwitting devil worshipers,) was overshadowed by the pointlessness of it all.
I bought my niece a Kindle that just arrived and I'm about to load it up with books to give it to her tomorrow for her birthday. I've decided to be a sneaky uncle and include good books that can teach better abilities to think or at least to consider science cool and interesting. She is currently in the 4th Grade with 5th coming after the Summer.
She reads basically at her own grade level so while I'm open to stuffing the Kindle with books to be read when she's ready, I'd like to focus on giving her books she can read now. Ender's Game will be on there most likely. Game of Thrones will not.
What books would you give a youngling? Her interests currently trend toward the young mystery section, Hardy Boys and the like, but in my experience she is very open to trying new books with particular interest in YA fantasy but not much interest in Sci Fi (if I'm doing any other optimizing this year, I'll try to change her opinion on Sci Fi).