We started (and finished) this wonderful little book yesterday. It's a quick but absorbing read that includes humor, suspense, children who act like real children, and the reality of Nazi regime evil without taking that aspect so far that it's inappropriate for younger children.
My girls loved the courageous and lively nun, Sister Gabriel, who counts on the twenty Christian boys and girls to take in the ten Jewish children in need of refuge. And they were pulled in to the story immediately through the 13-year-old narrator, Janet, and her brave and clever friend, Henry.
I won't tell you more, so as not to spoil it for you, but I highly recommend Twenty and Ten.