Firstly I would establish your maximum mileage a day, we find that 250 is ideal and much over 350 gets a bit dull, obviously depends on road, car and children etc. as this is key to your planning (E.G. Calais to Saint Tropez is roughly 3 X 250 miles)
We usually make a vague plan, and book hotels as we go, and sometimes stay a couple of nights if we like an area, although I appreciate that may be more difficult with children.
Reims is okay, but I much prefer Dijon for walking around / eating etc. Lyon is nice in the centre,
I would try an AI agent for the planning, it usually works well
"We are driving from Calais to Saint Tropez, with children aged X and X, and would like to drive no more than 250 miles a day, stopping in some scenic towns and cities that might also have some child friendly attractions such as water parks etc. can you suggest a route, please"
I would also recommend getting a toll transponder (bip and go) as it makes life much easier on the motorways and some car parks.
...and I assume you're not planning to do this in August...!