Pratchett's good. I like him. And I started Narnia about 4 years ago. I was a teacher aid in my senior year for a 3rd grade teacher and the teacher didn't always have something for me to....But the Narnia books were in there, and since I sat next to the bookshelf.....I just couldn't really get into the book. I didn't even get that far. Maybe 30 pages? I don't know...And yes, I do intend on re-reading it after I finish the others.
And as for kids and reading. I think just instilling in them a love for reading a lot of different types--at least, not just humor--will make it easier for them to enjoy stuff like Pratchett
and Lewis. Even if their styles are completely different, they're both really good, and they both write/wrote predominantly fantasy (to my knowledge).
On a side note, I really need to focus on reading one book at a time. I whipped out Card's Ender books a couple days and so I'm re-reading them on top of Narnia....And I have a craving to re-read Ella Enchanted, too....