We did this with a touring caravan for years...ours was a fairly luxury model so I am always embarrassed to describe as camping but principles the same.
L Garangoire is a fabulous site. The activities for children always used to be amazing and they used to offer free children's clubs on an evening if parents were eating in the reason,y priced restaurant. Our kids simply loved this site.
Eurocamp had a portion of the site. Were expensive but they put in a lot of their own activities.
This type of holiday for younger kids and parents was simply amazing and ours have some of their best memories going to sites like this. The key is the travel down and back and with tents, overnight stays en route are more tricky, although I think Euro camping etc can sometimes accommodate one nighters.
Driving in France using toll roads is a delight. Rarely traffic james, except Paris and Bordeaux. You can blitz long distances doing 130km per hour speed if you have the car for it. Lots of Aires and service stations
The Vendee we found to vary between warm and sunny, but also a lot of mild and rainy days. The rule we followed was anything south of La-Rochelle and the weather was pretty consistent of warm and dry.
You describe sites by their group names, like Yelloh. Don't disagree with others views but be aware some of them move groups, ie could be a Castel and more to Yelloh. However, we did our research and we never stayed at a bad site.
We tend to fly a lot these days but I look back on our camping in France and northern Spain with very fond memories of the long relaxed days and kids happy and entertained. Would recommend