3 Days in Brittany

Stay in Dinan to see St-Malo, Cancale,( oysters!) MSM.
Or stay on the other side, Vannes or a village nearby with fabulous oysters.

My instinct would be to suggest taking the train to Quimper and then working your way back along the southern coast, but of course that's still awfully pressed for time. You'd have options like Concarneau, Carnac, Quiberon, Vannes, and Rochefort-en-Terre.

If you're continuing the drive toward Bergerac, your children might enjoy Nantes for the Machines of the Isle of Nantes. The problem is just that there is so much to see!

Brittany is a huge region, with many areas to choose. My thoughts are similar to Amy’s. Train to Quimper, tour the town and then pick up your car. If you want a base for 3 nights, consider the Lorient area right on the water, and central for the Morbihan région. You can visit the towns Amy mentioned from there.

If you and your children would like to do a bit of boating, there is a great program at the Eric Tabarly center: https://www.citevoile-tabarly.com/en

There are also ferries from Lorient for island exploring: Groix is very close with beaches and history, Belle Ile lives up to its name of beautiful island. I’m suggesting these water oriented options, because you will be spending more of your vacation inland (Bergerac). And Brittany’s seacoast is extraordinary.

You could also day-trip to Carnac, or stop there on the way east toward Vannes and Nantes.

I enjoyed the area around Dinan, St. Malo, Cancale and MSM.

I also enjoyed the area around Vannes, Auray, Carnac, Bay of Morbihan.

I'd approach it by taking a train as close to MSM as possible, Rennes or Caen, picking up a rental car and spending a day and a half north and another day and a half south.

I don't think you have time to go further west on the peninsula.

In August, there will be towns having pardons. It's a local Breton custom that includes traditional dress, food, music. It would be nice to catch when you're in the area. They go throughout the summer months, different towns on different weekends.

Do the kids want a beachfront hotel, say, in Dinard? There's a steam train excursion, maybe near Quimper. I'd want air conditioning in August.

From the airport CDG there are direct TGV trains to Rennes, not to Quimper or other cities in Brittany. From Rennes it is shorter to reach St Malo than Quimper (more trains too). Anyway both areas are beautiful and worth seeing, with picturesque towns. The north coast in Brittany is usually wilder if you enjoy rugged coastline, cliffs. The south coast has longer sandy beaches if you are looking for that. On both sides you will find seafood.