What's the best place to spend a month with my 4-yr-old son?

If it were me, I would not take my 4 year old to a big city but rather on a beach. I would choose the Atlantic coast, and a good beach safe for kids, e.g. La Baule. I'm sure they have some carousels there as well. YOu did not say what time of the year that would be.
The mountains are also fine, and for instance in Annecy you have both swimming and beaches on the lake and mountain walks.
(and also carousels)
As for Paris, once you've seen the carousels and gone on the Seine for a ride on the bateaux mouches, and in some gardens with kids' playgrounds... there isn't much else for kids apart from pollution.

Good luck.

I'm wondering if there is any way he could be in a playgroup (like part-time in a crèche) and interact with little French kids, and pick up some French in the process(with astonishing speed). Or if not, why not a gîte or Village vacances familles (for family holidays). Some are close by towns or cities so you can soak up some urban culture too - one nice thing in France is the street events in the summertime. Your son will meet a lot of French kids at those places.

http://valvvf.eu/ is one branch - vvf-vacances.fr/ another.

Fully agree with "finna"

I don't fully agree, as mamalinda will want her son and herself to get access to an experience that is somehow "typically French" and in any event something they couldn't get in their home country, wherever it is. This could be in a town, city or the countryside, but it should provide some exposure to French ways of life.

La Baule is nice, but might be a little expensive. You also might consider Ile Noirmoutier.

You also might consider the city of Angers. There is a Chateau, and old part of the city, beautiful gardens and of course Carousels. Here are some pictures.

A beach sounds nice but I would suggest Annecy. there is a beautiful park with a carousel for him to play in, and they have lots of strange unusual ducks in the lake for him to throw bread to. You could also swim at Annecy and hire one of those water pedal bikes on the lake.

Lagatta, you are right regarding adults, but come on... at 4 years old, do you really think that the little boy will want a typically French experience and enjoy the French way of life at HIS age ?

In Paris, he would want to play, he will want the zoo, going up the Eiffel Tower, and that's it. I doubt whether he will really care whether he's in Paris or Pataouchnoki as slong as his mum is with him and he gets the occasional ice cream.

I'm not thinking of museums or "cultural events" - I'm talking about interacting with people - notably kids - who have a different way of life and speak a different language.

I did NOT suggest a month in Paris. I suggested typical French FAMILY holidays.

Hello everyone,

Thanks so much for all the ideas and suggestions. We just got an offer to stay with a family just outside of Trets. Is anyone familiar with that town? Do you think it would be a nice place to stay for a month? And would it be better to go in May/June, or in September?

Thanks so much for your help, linda