Oh hi thar. I was just scanning the blogs (once in a blue moon thing now) and the name "C. S. Lewis" jumped out at me; I'm a huge fan.
In answer to your question about the Narnia series being for kids, the answer is yes and no. Lewis originally wrote them as kids' books, but they really transcend ages and are wonderful for any and everyone. Lewis packs alot of truth into his works, and each one is really a work of art. So yes, be sure to read them.
As to the order, the only thing I would say would be to be sure to start with The Lion, the Witch, and the Wardrobe first, followed by Prince Caspian, The Voyage of the Dawn Treader, The Silver Chair, and The Last Battle. The Magician's Nephew is a prequel of sorts, telling the history of Narnia, and The Horse and his Boy aren't part of the main storyline—it takes place sortof in the middle and is more of a side adventure. So as long as you start with The Lion, the Witch, and the Wardrobe and read those first five in order, you're good to go.
Eeko mentioned Lewis' Space Trilogy; I'm actually half way through them myself (got the books for Christmas), and thus far all I've had to say is "wow." The pure creative genius Lewis exhibits is extraordinary—he creates whole worlds from (what seems to be) scratch. Truly marvelous.
And as to The Screwtape Letters: all I have to say is yes, Yes, Yes! This work is one of his best by far—so insightful about the Christian walk and how demons/temptation relate to us. Wonderful stuff here.
Keep up the good reading!
Also man, please take a look at my latest MoC if you like: clicketh