Not sure, if they are supposed to be in German or in English. If German - I agree with Michael Ende. Momo is a great story also for adults. Anything by Erich Kaestner. Fliegende Klassenzimmer, Doppelte Lottchen.

My father used to read Wilhelm Matthießen: Das Rote U, which he had read as a Child. Gripping story.  (would have been read in the 30ies)

Timm Thaler (also known as The Legend of Tim Tyler: The Boy Who Lost His Laugh)
http://en.wikipedia.org/wiki/Timm_Thaler_(1979_TV_miniseries)

Great and scary story

Die rote Zora und ihre Bande: Kurt Held (would have been read in the 30ies)

Die Kinder aus Nr. 67: Lisa Tetzner

Als Hitler das rosa Kaninchen stahl: Judith Kerr (long version!)

Das kleine Gespenst: Otfried Preußler (one of my personal favorites as a child)

But most of these books have not been translated into English. However, with some basic German, some of them are easily readable for some one with maybe one or two years of German at 11 and they are rewarding enough in themselves.

Good luck

some german female artists:

http://www.amazon.com/Gabriele-Munter-Search-Expression-1906-1917/dp/1903470293