TC
Here's where we've been and thoughts (obv. massive generalisation, but here goes)
Seaside:
Brittany - Like Cornwall. Similar weather.
Vendee - Better weather, still not 100% guaranteed. Bigger beaches, less cliffs and coves than Brittany. More water parks etc. Islands (Re and Oloron) no parks but best for young kids imo. (technically in Poitou Charentes).
Languedoc - Busier, esp. in French peak, but not as busy as Cote D'Azur. if you pick the right areas probably the best bet in France for a guaranteed sun beach summer hol. Also great red wine if you look for it, probably best value up and coming region in France.
Inland:
Dordogne - a bit of a cliche, but popular for a reason. Lovely countryside and plenty to do. Not as crowded as the coast, especially away from the Tourist hotspots.
Loire - beatiful chateux and wine, not so much for kids.
Alsace - Often overlooked, but gorgeous mountains and wine etc. Strasbourg is lovely, though again not so much for kids.
A few other random thoughts:
This www.france4families.com/ is one of the best general websites for kids & France.
If you're looking at Languedoc thiswww.creme-de-languedoc.com/index.php is a cracking website.
Driving generally much better than UK, just avoid Paris and busy days. www.bison-fute.equipement.gouv.fr/diri/Accueil.do is the French Gov MWay website, again very good.
Don't overlook the motorail, not cheap, but just affordable if you do one way (driving down and motorail back is the best way round to do it) www.raileurope.co.uk
If you don't HAVE to have a sea beach than river and especially lake bathing in France can be fantastic for kids. There's a roped off clean area, often with a little sandy 'beach'. Sometimes boat hire, playgrounds etc. etc. Get yourself a Michelin Atlas, look for lakes that have a blue circle on them (Baignade) and Google 'Baignade + (the nearest village)' making sure you put 'French' in advanced settings.
Don't know your kids ages but we've found both caneoing and those hire wire forest zip things (many French ones have kids courses from 3/4+) to be surprise bonus hits with our DDs.
FINALLY - If you are looking for a halfway stop to Languedoc we found this one this year. Lovely little hotel, modern but not ultra-trendy (aka stuck up), very family friendly (run by a young French/Swiss couple with small kids), great food, not too expensive, 15 minutes off the Mway and 6-7 hours from Calais, 4-6 hours from the coast.www.aucoeurdemeaulne.com/uk/index.html
Good luck, and enjoy France - we love it!!