Have a look at Camping Des Menhirs at Carnac. Eurocamp, Keycamp, etc all go there but the site has its own caravans and chalets to rent too and they looked to be good quality (unlike some site owned caravans I've seen). It has pools, various sports facilities, bar, take away, entertainments etc but also has the advantage of being 5 minutes walk from both the beach and the centre of Carnac Plage. There's lots to see and do in the area and I would highly recommend it. We were there in 2006 with a 2 year old and a nearly 6 year old but there was plenty for all ages.

Can't add any more Elaine has pinpointed it spot on
You may find better deals going independantly but it can be a hassle

yep we went to des menhirs,last year it was a very good base for a family holiday,loads to do and see in the area.we loved it,my kids loved the zoo at pont scorff,

The area around the Morbihan gulf, places like Penvins, etc, are real nice too, but i can't recommend a campsite as we stayed in a villa!