Put our kids in kindergarten and first grade when we lived in Germany. Zero language problems and no learning problems. We used English kids books and some home schooling materials to fill in the English/American gaps on the weekends. Sadly, we didn't keep their skills up after our return either.

I'm sure you've done some research but theirs is a track school system in later years. If not, wikipedia is your friend: Education in Germany - Wikipedia, the free encyclopedia

It would also be great for their German classmates, since they would have a built-in english and slang tutor.

Viel Glück!!