Two weeks in France

That is very realistic; you just have to narrow down where you want to go in the broad area of the south of France

Thank you. We are hoping to soak up the atmosphere of just the two areas and not jump all over the place for two weeks.

Very realistic. You might be prepared for a few more Paris day trips if you feel you need to get out of the city a bit more. Versailles, Chartres, Fountainbleu might be options.

Sounds like a very realistic plan. You may want to consider Chartres, a short train ride from Paris. We spent two nights on our way to the South of France. We especially enjoyed the evening light show on the facade of the cathedral. It was incredible. I think one night in Chartres is plenty of time to enjoy this charming city. Your twins would probably love the light show! We were there in the month of September and it was perfect.

How many full days will you have in Paris? Not counting arrival / departure days.

I wouldn’t add more than 1 (maybe 2 if you have 7 full days) day trip to your time in Paris. There are so many fun things to do in Paris that not even a week is enough for all the sites, not to mention wandering and exploring which is the most fun imo.

Don’t miss a little time in the Luxembourg Gardens. A beautiful park in the heart of Paris.

Consider an open-jaw air ticket, flying home from Marseilles other airport.

You'll want a car the second half of your trip.

Adam's suggestion is useful. Multi-city routes have to be obtained with a multi-destination search function but can save time and even money. I'd reverse direction, flying into Marseille Provence airport to start, working your way by train north to Paris, and flying home with a much more comfortable departure time than if you were connecting from Provence.

Avignon, Aix-en-Provence or Marseille are all useful bases in the south that are easily accessed by public transport from the airport. If you want to get deeper into the countryside you could rent a car for a day or two. I think the average 12-year-old will be more impressed with the great castle overlooking a mighty river in Avignon than the lily pond at Giverny, but you know your kids.

I think your concept is perfect. Two places is just right. September was beautiful in Paris but still kind of warm in much of Provence (what people often mean when they say "South of France"). I hope you have a great time.

What do your 12 year olds like to do ?

You could go to the Arles/Avignon/Cassis area--

Kayak and hike the Calanques of Cassis
Snorkel trip in Cassis
Kayak through the Pont du Gard

Make sure you do the bike tour of Giverny.

If you can do it open jaw, do so. If you need to do a round trip in and out of Paris then I would go to the south on arrival e.g. get a flight to the south about 4 hours after arrival and spend your last week in Paris. It is unpleasant to waste the second to last day of a nice vacation rushing back to the port of departure. Waste the miserable first jet lag day getting to your furthest destination. Depending on your destination, a train might be the best choice.

If it were me I would do the Dordogne rather than the Riviera. Canoeing on the Dordogne river, visitng the prehistoric caves, going to the chateaux and villages in this area, maybe a little hiking -- this is a wonderful area and IMHO more interesting and fun than the Riviera (which also has much to recommend it). You could train to Bordeaux, spend a night there and pick up a car the next day. I'd rent a place as a base. We have rented twice for a week each time with this person with good luck.
http://www.perfectionfortwo.com
We stayed last in Caducienne with another couple -- it was a wonderful little town and a beautiful cottage with two master suites, a garden where we ate dinner most evenings, US washer and dryer. Perfect for a family of 4 -- one of the bedrooms has twin beds, both have ensuite bathrooms with nice showers. The village is charming and has a classic old abbey, a bakery, an ice cream shop and a restaurant and cafe. There are supermarkets in nearby towns; we picked up groceries when out and about and visited a wonderful weekly market. There is a small one in the village as well. We used it as a base in early September to visit sites in the area. Just wonderful.
There are a couple of things we visited here, although not much on the town of Cadouin itself.
https://janettravels.wordpress.com/category/south-of-france/

A week in the south and a week in Paris is a perfect way IMHO to structure a first trip to these areas and September is the best month IMHO for France. It is the most popular and expensive month for Paris, so nail down lodging there asap.

I agree with Susan that you don't need a lot of day trips with a week in Paris. We were there for eight days and only took one day-trip (Versailles), although of course it depends on your daughters' interests.

Caducienne was the name of the cottage; the town was Cadouin which is mentioned in the post. Caducienne followed the link to the cottages -- but was perhaps not entirely clear that it referred to that.