Personally, I think your kids may be a bit young to appreciate the D-Day sites, but you know their interests better than anyone else. .
Reflecting on trips we made when our sons were that age, I would vote for Burgundy (Bourgogne). Some places you may want to consider would be:
Provins, east-southeast of Paris -- an impressive walled town with a lot of medieval-themed entertainment geared toward your kids' age. You could pair that up with a visit to Fontainbleau or Vaux-le-Vicomte, or both. The first is a predecessor to Versailles and the second a private château that served as a stand-in for Versailles for the film The Man in the Iron Mask.
Semur-en-Auxois -- another medieval town with impressive towers and ramparts you can walk. Not a destination, in my opinion, but a good place to stop for an hour or a few hours.
Château de Guedelon -- a new castle in the process of being built using only materials and building techniques available to medieval builders. I suspect children your kids' age would be very interested in learning more about how these huge structures were built.
Château de la Rochepot -- a privately-owned castle that seemed to appeal to the families with kids that were there when we visited
The Hospices of Beaune -- a hospital for the dying that is pretty impressive on its own, plus the display of medical equipment from the era (which look like stuff you would buy at a hardware store) might make them appreciate modern medicine a bit more. While in Beaune, you could arrange a bike trip to local vineyards through Bourgogne Evasions. They have adult and kid-sized bikes and leave from a park just west of central Beaune (Parc de la Bouzaise). It's an easy ride with beautiful scenery.
If you're interested in seeing Roman ruins, some places you can stop along the way to Chamonix would be Autun, Lyon, and/or Vienne.
Pérouges is a medieval town that kids probably would enjoy
It might be a little out of your way, but if you have a chance, take road D531 along the Bourne river through the Gorges de La Bourne. The cliffs, rock overhangs, and the twists and turns are awesome and is something your kids likely will long remember. Google Gorges de la Bourne and look for pictures and videos to get a sense of it.
And of course, Chamonix is a great base for sightseeing and hiking. I assume you know all about that, as you've made it your destination.