We stayed on that site last summer with Eurocamp and it was great. The beach is a 2 minute walk and the centre of the resort is 5 minutes walk. There is a SuperU supermarket a few minutes walk away. Carnac Plage is very much geared up to tourists and there are plenty of places to get meals, although we didn't find anywhere selling traditional French food - it was mainly pizzas and steaks. Carnac and Carnac Plage are very good for cycling as the roads are quite narrow and in the summer get very busy so you can use the cycle lanes and paths to avoid the jams.
Our daughters were 2 1/2 and almost 6 when we were there and we managed to explore quite a lot of the region - Quiberon, Auray, Vannes, etc. Our only problem was the day we took them to the aquarium at Vannes. When we got there we discovered that most of the car parks had a height restriction barrier of 1.85m. I had a roofbox on my car and that takes the height of my car up to 1.96m. We got some very strange looks as we removed the box, hubby carried it under the barrier as I drove through and we then put the box back on to leave the car.
I don't know if you are travelling with younger children but the Fun Station Kids Club run by Eurocamp and Keycamp for 4 year olds + was great for our older daughter. In the morning they tended to have crafts and board games and the afternoon was more active things like football, although some mornings they were around the site doing treasure hunts etc.
I'd thoroughly recommend Carnac and this campsite.
