MabMama and philomom, I love your suggestions. I also have a box of favorites back in Germany.
I've donated lots of books to public libraries and to our elementary school library, both in the U. S. and in Germany.
Being expats here, I've given a lot of our German books to my children's international school. (You'll find English children's books on a regular basis here, but anything else is really hard.)
Depending on your children's ages and developmental levels, it might also work to swap some with a friend. (I just did that here for the above reasons - those are "first readers" in German my youngest one will be ready for in about a year. After he's done with them they'll leave the house.)