Reddit mentions of George's Secret Key to the Universe

She was looking for this, this, or possibly this.

Blue Balliett's series:

1. Chasing Vermeer
   
   
2. The Calder Game
   
   
3. The Wright 3
   
   Stephen Hawking's series:
   
   
4. George's Secret Key to the Universe
   
   
5. George's Cosmic Treasure Hunt
   
   
6. George and the Big Bang
   
   
7. George and the Unbreakable Code
   
   Others...
   
   The School Story (It has two female leads, not sure if that will be a deterrent for your nephews)
   
   Einstein Anderson series
   
   Encyclopedia Brown series
   
   Hardy Boys series
   
   Sammy Keyes series
   
   Check the ages. All of these are above a 5-year-old, unless you're reading to him. Most of what you're asking for will end up being detective mysteries.
   
   I wouldn't recommend nonfiction unless the subject matter matches their real-world interests... or you happen to know they prefer non-fiction.
   
   

You can [adopt a star] (http://adoptastar.whitedwarf.org/) for her. Just be aware that this isn't an ACTUAL adoption/naming of a star, and it won't be listed with NASA or anything. It's just a nice novelty, but at her age, she'd be pumped.

If you haven't already, check out the [NASA Kids Club] (http://www.nasa.gov/audience/forkids/kidsclub/flash/#.VLIU5SvF-So) site. Lots of space-related activities and education!

You can buy her a [Meteorite kit] (http://www.amazon.com/gp/product/B0039ZT0KY/ref=as_li_ss_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B0039ZT0KY&linkCode=as2&tag=ufowioutrspcgft2012-20) that also provides educational materials as well as meteorite samples

[Here's] (https://www.etsy.com/uk/listing/196434533/women-in-science-valentina-tereshkova?ref=related-3) a great poster of Valentina Tereshkova, the first woman in space, to keep her enthused about space exploration.

And [here's] (https://www.etsy.com/listing/156254584/space-explorer-iron-on-patch) a cute iron-on patch to make her feel more official!

There are also a lot of great children's books about space, such as [You Are Stardust] (http://www.indiebound.org/book/9781926973357) and [Star Stuff: Carl Sagan and the Mysteries of the Cosmos] (http://www.amazon.com/Star-Stuff-Sagan-Mysteries-Cosmos/dp/1596439602/ref=sr_1_3?ie=UTF8&qid=1420958032&sr=8-3&keywords=cosmos+carl+sagan+book). Stephen Hawking also wrote a series of [children's books] (http://www.amazon.com/gp/product/1416985840/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1416985840&linkCode=as2&tag=stephenhaw042-20) with his daughter Lucy that are incredibly adorable. And finally, [Born With A Bang: The Universe Tells Our Cosmic Story] (http://www.amazon.com/Born-Bang-Universe-Sharing-Children/dp/1584690321/) helps tell the tale of the beginning of our universe, narrated by the Universe itself!

Of course, there are also many neat planetariums on the market. She can have a star show every night! Invest in a telescope (if you haven't already), and take her out on a clear night to explore the stars with her own eyes.

It's really great that you're encouraging her in this. It's vital for future space exploration that we keep children's interest piqued when it comes to science and space. Heck, she may end up being the first woman on Mars! Kudos to you, and best of luck finding something perfect for your little space explorer, and I hope she feels better soon! :)

Quick... http://www.amazon.com/Georges-Secret-Universe-Stephen-Hawking/dp/1416985840/ref=sr_1_10?ie=UTF8&qid=1293078262&sr=8-10

Stephen Hawkings has written a children's book series along with his daughter. My 6yo son really enjoyed these, but there's definitely enough science there for older kids too. Black holes, galactic travel and a kid's misadventures. George's Secret Key to the Universe