@mrs.smooshy

That's a great question! I think that there may be varying reading levels but not by much because the Chronicles of Narnia is  a series as a whole so it wouldn't make sense to have different reading levels. Though, I do think that "The Lion, The Witch and the Wardrobe" may be slightly easier than the rest of the books and is made for children a bit younger. Also, I think that "The Last Battle" and  "The Silver Chair" is not made for younger children because the book itself has the themes of grief and lessons that you do not really quite understand when you are really young.

Like you mentioned, the dialect of the Pevensies in their Golden Age is a little hard to understand when reading it to kids, but it may be put just like that so the kids reading the books may understand that they were more mature and used more of a formal talk symbolizing how much they have grown up in Narnia. 

Although there is only a 6 year period between the publication of the first book and the last book, I do feel that Lewis writing style does change somewhat as the books progress, from the whimsical fairytale style of LWW in 1950, to the more weighty themes of apocalypse in The Last Battle in 1956. Things definitely get more serious from book to book.

There is also something unique about HAHB which sets it apart from the other stories, and that is that it doesn't have a child from our world to serve as an audience Point-of-View character. As a result there is just something fundamentally different about the dynamic of this book, as the lead characters aren't able to provide that contextual reference back to our world.

For what it's worth, I always hated this book as a child. Unlike every other book in the series which I read countless times as a child, I don't think I ever read HAHB all the way through untill I reached adulthood and even now it's still my least favourite entry in the series.

It's clear that Lewis's work evolved as the series progressed. I'd definitely put the so-called "Pevensie Trilogy" (LWW, PC, and VODT) down at the lowest level agewise. The Silver Chair and the Last Battle are both a bit darker and more advanced, as Icarus and Edmund pointed out. 

The Horse and His Boy isn't quite as clear about its reading level. I wonder if the politics of the story are among the things that were confusing your children (just an uninformed guess,  mind you.) The lofty speech of the Calormens may also prove a bit hard to grasp for younger readers. 

The idea that different books in the series are best for different age groups is also possibly supported by the fact that Lewis took the liberty of inserting one or two occasions of mild swearing in the Silver Chair and the Magician's Nephew.

I wonder also  if maybe Lewis intentionally raised the reading level of the books as he went on for the benefit of kids who had started reading the Narnia stories at a younger age and would now be able to keep reading them comfortably. 

Posted by: @the-mad-poet

It's clear that Lewis's work evolved as the series progressed. I'd definitely put the so-called "Pevensie Trilogy" (LWW, PC, and VODT) down at the lowest level agewise. The Silver Chair and the Last Battle are both a bit darker and more advanced, as Icarus and Edmund pointed out. 

The Horse and His Boy isn't quite as clear about its reading level. I wonder if the politics of the story are among the things that were confusing your children (just an uninformed guess,  mind you.) The lofty speech of the Calormens may also prove a bit hard to grasp for younger readers. 

The idea that different books in the series are best for different age groups is also possibly supported by the fact that Lewis took the liberty of inserting one or two occasions of mild swearing in the Silver Chair and the Magician's Nephew.

I wonder also  if maybe Lewis intentionally raised the reading level of the books as he went on for the benefit of kids who had started reading the Narnia stories at a younger age and would now be able to keep reading them comfortably. 

  

This is my theory as well.  When I was a child I was a very big fan of The Black Stallion series and the author had a fascination with Arab culture which featured strongly in several of the many installments.  And I'm not sure where, but I had read other books loosely based on the Ottoman culture as well.  It wasn't brand new to me and I had found it very appealing.