I have travelled in late fall, end of September to late November and mid to late winter (March). Last fall, I arrived in Paris on Nov 2 and departed Nov 23. There were still MANY tourists around. The lineup for the Eiffel Tower and the Louvre were still considerable in mid-November. For this reason, I would suggest off season travel. In addition, I do not like hot weather; many rentals will not have air conditioning.
I have visited the Nice area from late September, into October and another visit in Mid-March. A bit of rain in March, but nothing to stop me seeing the sites. I stay in Villefranche-sur-Mer when there; the town is on a hill, so pay attention to where the lodgings are. I prefer the old town, closer to the water, train and not far from the bus.
In Paris, beginning in mid-November, the Champs-Elysées hosts a Christmas market. Delicious mulled wine. It was a lot of fun to roam down the avenue, watch people and sip wine. I have the plastic cup from the wine in my bathroom; a daily reminder of Paris. The weather was fine; a few days were balmy and shirtsleeve weather by mid-morning. I brought several layers and thin mittens to keep warm; no problem. Scarves are the travellers friends. It is very easy to shed layers and put them in your day bag. Paris has so many small parks to picnic in. I love the markets; I think the kids would enjoy all the fresh fruit, veggies and the whole fish and chickens with the heads and feet on.
Enjoy France.