This is really two trips – one in southern France and one in northern France. In southern France, you can’t do better than the castle of Carcassonne, which will take a whole exhausting day and load you down with plastic swords, plastic suits of armor, cardboard crowns, and all kinds of medieval junk. But it’s a huge, beautiful castle, and the kids can climb all over it.
The next day, a short drive takes you to Toulouse, which is a beautiful town to walk around. It also has a lovely Romanesque church to visit. In the afternoon, or on the next day, go to see the Romanesque cloister at Moissac. My kids loved the square in front of the cloister where we had lunch.