A little girl who lives on the moon with her parents goes out for a walk. She ends up getting recruited to save galaxy. Once she's done, she is returned home through time travel and is shown to have been gone for only 5 minutes.
The plot is vaguely similar to Heinlein's Have Spacesuit, Will Travel. The little girl is a secondary protagonist, but some of the action happens on the Moon and Pluto, and the protagonists do save, not the galaxy, but the human race. And in the end they are returned after weeks of adventures to a time shortly after they left, to their surprise. "Return them forthwith, to the space/time from whence they came."