If they haven't read the Harry Potter series all of them have been translated.  Ditto with the Paolini series.

Bowdlerized "child-safe" editions of Grimm's Fairy Tales are easily found.  Can hardly get more German.

Astrid Lindgren's books have been translated into German, tho' you wouldn't know that if you searched Amazon using "Pippi Longstocking (German edition.)"  

Powell's Books in Portland had used children's books in good condition in German the last time I was in there (2002.)  Might be worth an email asking them for a list and prices.  

Taschen has an outlet in New York City.  No idea what they have.

The market for books with a modern foreign language on one side and the translation on the facing page is too small for a publisher to bother with.  There might be CDs of German children's songs that have an insert with the German lyrics and translations available.

 

Ever since I learnt about confirmation bias I've started seeing it everywhere