Mont St Michel, please have mercy.

Walker, I was thinking Sancerre or is that Sancer. :-)

Two spellings and a thousand pronunciations! :)

Not so fast, my friend (Lee Corso).

You've got to be very careful if you don't know where you are going because you might not get there(Yogi Berra).

I would substitute Paris and surroundings, one of the world's great cities, for much of Belgium. Compare Paris with Brussels. Brussels is best known for a little boy "taking a leak" in the main square. For Paris your sons can tell their friends that they climbed the Eiffel Tower, the Notre Dame cathedral, and the Arc de Triumph. Bruges is nice, but Colmar is Bruges without the waffles and mussels. I have not been to Ghent since 1964, but all remember is some large dogs. You can buy Stella Artois at Costco.

So think about landing in Dusseldorf, taking the train from the airport to Paris (about 4 hours) and sleeping in Paris for your first night. After a few days in Paris you can rent a car and tour Burgundy and Alsace, turning in the car in Strasbourg or drop off in Dusseldorf paying a couple of hundred $ drop off fee.
OR
After Paris, take the TVG to Avignon, rent a car, tour Provence and make your way north touring the French Alps, Burgundy, and Alsace.

Both of these "flow". Its only "dough".

<<Bruges is nice, but Colmar is Bruges without the waffles and mussels.>>

Yes, Paris looms large in any itinerary I try and work out. Your idea of Paris/Avignon and driving up to Alsace strikes me as excellent, thanks. It definitely could have the flow, now just to look into that other thing. :)

Every choice I make means I'll be giving something else up, but Stella Artois isn't one of them! :)

Thanks for all the input from everyone, it really did help me out. However this is now an ex-itinerary as I just purchased a connecting flight Dusseldorf - Lyon and will be hitting Provence/Dordogne after all.

I had to give up a small country to do it, but what's new.

Last year I took my 2 grandsons(and their parents), each 2 years younger than your sons, to the Dordogne,Provence, and Paris( one night stops in Carcassonne and Rocamador).The boys loved the trip.
So what is your new itinerary?

Well, I'm working on that, but the basic idea is 3 days Auvergne, 6 days Dordogne, 3 days Languedoc, and 6 days Provence. I'll start a new thread when I get a day by day plan together. Thanks.