A few random thoughts-
Next summer finds the Olympics in Paris, which will add another dimension of crowding and lodging inflation.
I love starting in Venice- what is not to love, especially for the kids?
For starters,
I'd suggest Row Venice, small group rowing lessons on the back canals, 90 minutes, about 100 euros total, for the group.
Venice to Lake Como works for us, we just did that trip this past May. We stayed in Varenna, and rode the ferries to Bellagio and Menaggio.
From Varenna I'd go to Tirano, Italy, and catch the remarkable Bernini Express train, using the regional trains,( not the panoramic cars), which gives you more schedule options and is less crowded, and runs on the same tracks.
Spend the night in Pontressina or Chur, then onto the Berner Oberland for as many days as possible. The kids will love riding the gondolas, cogwheel trains, lifts, to the most spectacular scenery in the world.
From the BO to Lucerne , one of the most charming cities in Switzerland.
Then onto Strasbourg/ Colmar.
I'm no help for the rest of France.
Have a wonderful family trip!