Hi Phil
SW France is a VERY BIG area - do you have any preferences? Do you mean right in the South West, on the West Coast (Biarritz area), do you like inland or beaches, or do you mean the western end of the Mediterranean? The weather tends to be more reliable around the Mediterranean, because of the westerly winds dumping more rain on the Atlantic coast. Some people consider the Dordogne to be in SW France - this is a lovely area, if you like 'inland', as is the Cevennes and Tarn Gorges(part of the Massif Central) and the Ardeche.
What ages are your children - this might affect the sites people recommend. I'm sure loads of us can come up with suggestions and recommendations if you can be just a bit clearer on the particular areas you are looking at. You'll also get suggestions of the best routes, and overnight stops, once you're clearer on which particular part you want to head for.