I see you've already booked, but for anyone else reading, we've been (me and 2 DC, starting when youngest was a baby. Oldest is now 14) to:
Brittany (lots of times!!) Weather in the North can be a bit English for comfort, weather in the South's quite a bit warmer. It's nice- you stand a good chance of stumbling over a Fest Noz (traditional music and dancing) in the summer- they have them everywhere. The Cote du Granit Rose is lovely, and I really like Vannes. For me it's a bit too much like home in Cornwall though, I prefer somewhere a bit more traditionally French.
Nantes: really interesting, vibrant city. Reminded me a lot of Bristol. Check out Les Machines de l'ile!
Normandy: sorry Normans, I'm not really a fan (that'll serve you right for conquering us 😆) Find it a bit meh.
Loire Valley: excellent. Not just chateaux (although they are very beautiful). Seems to have the ideal summer climate, lots of stunning historic towns to visit, it's flat and there are loads of cycling routes, boat trips, loads to do with the kids. We paddled our inflatable boat under the arches of the Chateau de Chenonceau!
The Auvergne: definitely off the beaten track as far as British holidaymakers are concerned! Which does mean a language barrier when playing with the other kids from the campsite. Lots of French walkers climbing/ taking cablecars in the beautiful mountains though. Lakes for beaches.
The Dordogne: some of it is a bit too bloody much on the beaten track re British tourists! Sarlat was wayyy too touristy. Loads of cool paleolithic sites and stunning caverns to visit though, including Lascaux.
Cote d'Azur: very beautiful, but very crowded. I think a holiday there in August would be too stressful.
We're on a very, very tight budget, so tend to stay at very simple campsites- no water parks for us. Our absolute favourite is Le Milin Kerhe in Brittany- you can borrow their kayaks and have a paddle on their (very safe) river. I also recommend the municipal (actually intercommunal but same same) campsite in Chinon, right across the river from the castle.
French chain hotels are a godsend for when you're en route. They have well priced, decent family rooms.
I just asked my 8 year old what his absolute favourite place in France was, and he said La Vie Perchee, which is a Go Ape style place in Brittany 😆 (they have loads of that kind of thing in France, and much cheaper than Go Ape too).