Why would the next film be The Magician's Nephew? As I remember it, the book didn't feature the Pevensie children and isn't part of their story. If they were going to make a film of it, it should have been the first film because it sets up the story of Narnia, doesn't it?
I may be totally wrong and will happily stand corrected, I haven't read it for about 40 years!
You're right, it didn't feature them, they wanted to make LWW first because it's the most popular, but apparently now they are going to make MN.