We were staying at La Grande Métairie Campsite and, as this was a 13 minute walk from the site (and the number one restaurant in Carnac), it was the perfect place to go for dinner.
The food was fantastic, with a lovely atmosphere and the staff were very patient with our two boys, even though our six year old dropped his fork about 8 times through the meal.
My only complaints were, that on arrival we were early and (even though the restaurant was almost empty) the manager showed his displeasure at this fact. The second was the children's menu. We had been in France for a week and as our eldest had had chicken and chips for almost every meal, he chose the Galette. Unfortunately he didn't like it and it turned out that the nuggets and chips were fantastic (the youngest chose this).
Apart from that the food, wine and service was lovely and I highly recommend the restaurant.
One final warning, If you are staying at La Grande Métairie Campsite and you go for dinner, check what time they are closing the south entrance. Whilst it is possible to jump the gate, it isn't easy with two young children or a not very flexible wife. I would suggest going for lunch or driving there.