I could recommend plenty of books for that age-group, but so few of them have been translated. Michael Ende is good, and there are translations of The Neverending Story and Momo. Otfried Preussler's Krabat. These three are thrilling and all children I know find it difficult to make a pause while reading them.

I don't know why the 1930's, but most books by Erich Kästner ought to be translated.

Ask again when they have learned to read German. :-)