Of course, they will not actually feature in the novel. They are, however, an important background. The Hanni character will realise in that thread of the story. The girls will get some inkling when they learn about their teacher being interned. I shall put in the details about Clara Lehrs, but only as an appendix at the end of the book.
There is plenty of witness statements about the death camps. The BBC provides a lot, as does the University of Sussex.
Useful links include: http://www.rudyfoto.com/hol/campstory.html
I’ll now only be doing a little research over the coming week, returning to completing more in detail mid-September.