I've already read it, and in spanish its called "fugitivos!" I think it translates like "runaway" or something. Its about three children that are running away from the nazis. I think the cover (at least the cover of the book that I had, written in spanish), had the three kids on a cow. Thanks!

Or maybe I'm wrong and the book was written originally in spanish so it hasn't been translated to english? Don't know.
But it was probably written in german originally, just because is based on germany.