dd liked various Briefe von Felix books, scroll down to see the others:
www.amazon.de/Abenteuerliche-kleiner-erforscht-Planeten-Bilderb%C3%BCcher/dp/3815714001
She doesn't mind whether books are translated from Engish generally. When she was a beginning reader, she quite liked Nick Nase (easy-reader detective stories).
www.amazon.de/b?ie=UTF8&node=347120011
I'm not sure what level you are looking for though. She's reading a lot of the Shelti the Shetland Pony books at the moment, translated from English. She tends to discover some series and read the whole lot.
I see a lot of horse/pony books which are not translated from English. See this one and the dvd for it around a lot but we haven't read it:
www.amazon.de/H%C3%A4nde-weg-von-Mississippi-Ab/dp/3791504533
There's the equivalent of the Roman mysteries too, not translated, but that might still be too difficult for your dd.