Why did the Pevensie children die? (Last Battle Narnia)?

As per being the author, C.S. Lewis can do what he wants. An author can kill your favorite character in an instant. C.S. Lewis probably decided to do that to show what happens to those who die. You see, "The Chronicles of Narnia" is very similar to the story of Jesus (especially "The Lion, the Witch, and the Wardrobe"). When the 3 Pevensie children died, the went to the new Narnia, which is Heaven, or the new Earth, which has no evil. I don't know how old they were when they died, but I was quite upset also. But I'm glad you asked this! I was thinking about it myself for a long time. But I know, that when I die, I'll go to the new Narnia and live with Aslan, who portrays Jesus Christ, my Savior.