Flatland and Sphereland might fit the bill. Has she tried Tolkien yet? The Hobbit might be a good place to start.

ETA: Stephen Hawking has a series for children that's been reviewed well. I have my eye on these for DS in a few years:

http://www.amazon.ca/Georges-Secret-Universe-Stephen-Hawking/dp/1416985840

What is to give light must endure burning.