Until the last two books I have read, I realized I don't think I had ever read any fiction books on World War I. Now it just happened that I read two in a row. One adult book and one children's book. It was interesting to read about this difficult subject with two different audiences in mind. I highly recommend both books.
If you haven't read Ken Follett's historical fiction for adults, you should. It is quite an undertaking to read one of them, but it's worth it. His latest novel about WWI is called Fall of Giants.
I usually try to read the Newbery Award winners, and this year it was Moon Over Manifest, by Clare Vanderpool. Sometimes I read the Newbery books, and I don't actually think they're that great. This one I read and I absolutely loved.
You should try reading the adult book, have your child read the kids' book, and then compare notes. It might bring up some interesting discussion about an important time in history.