We did a Keycamp holiday in Normandy in June last year and had a lovely time. We need a short break before the end of Sept and I was thinking of 7 nights in Brittany in early Sept. It's MUCH cheaper to go in early Sept as opposed to late Aug but it'll be right at the end of the season and I'm a bit concerned things will be winding down and also that it'll be cold. Most of the parcs that I've looked at don't run kids' clubs or entertainment in Sept which is fine with us as we probably wouldn't take advantage anyway. And we'd go somewhere with an indoor pool and close to plenty of things to do if it does piss it down for a week.
French campsite can be fairly desolate in September, and the nights can be chilly. I would be tempted to look at a gite; Even ones with heated pools are much cheaper in September, and if you don't want a kids club then why be uncomfortable?
I'd also be tempted to head slightly further south for better weather. Get south of the Loire, particularly round La Rochelle has lovely weather in September
Have to agree. Being on a French campsite at the end of August can get pretty desolate as the shop shuts or goes to limited opening, the local supermarket might even shut (a lot are seasonal). It does also start to feel colder. If you really want a campsite, I would probably either go for a mobile home with heat or see if the campsite has any chalets/cottages. Or go for a Gite.