Nate,
Depending on the age of the children, Verdun isn't out of the question. Our two were 8 and 10 1/2 when we went. They were perceptive enough to understand a bit about WWI and the trenches. The fact that nature was allowed to retake the land was a good lesson in humility and hope, which they understood better than one might expect. Done well, a visit to Verdun could be a lesson in the futility of war, how the world can be repaired in spite of war, and hope for the future. It depends on how they are prepared for it, and doesn't have to be a long lecture, just a brief history of the conflict.
Mes deux centimes.....