Hi, we stayed at the Grande Metairie last year (2004). We took our own trailer tent - the emplacement was reasonably large and fairly flat, they were all divided by trees/shrubs. The showers and loos were always very clean, and even had baby baths!
The site itself was superb, the swimming pool was excellent, with good slides etc. Although I must admit that the weather wasn't brilliant when we went so we didn't use them much - too cold! The site had some animals on-site - the most popular for our kids was going to feed the donkey, which must by now be very fat as everyone else seemed to do the same! There was pony riding on-site, which you had to book in advance and was very popular, you had to lead the ponies around the site yourself, but they were a well behaved bunch so that was OK! The take away was very popular with us, the shop was very well stocked and had fresh bread and croissant every morning. We never tried the restaurant, preferring to head into Carnac itself for meals out.
They also had bike hire on site, we hired a bike trailer one day, to take our two year old twins out in which was great.
The playground was a little lacking, although my eight year old always seemed to find someone to play with up there so maybe that wasn't a problem. There wasn't much for the two years olds to play on though. There was a ball sports area aswell but with the little ones in tow we didn't get to use that.
They had entertainment most nights, from Karaoke (which unfortunately you could hear from our tent, why do people do it? Shouldn't someone tell them?

) to local small groups who were often excellent. The bar itself had a very laid back feel, and was next to the play area so you could bring the kids along and let them play whilst you enjoyed an evening drink. One night they brought in one of those bungee trampoline things, which all the kids absolutely loved! Apparently the site provided its own kids club, but we could never find out when and where it was - but maybe if you travel with an english company that wouldn't be a problem, although I would advise checking out the site's own accomodation etc. before you book - you may find it's a lot cheaper.
This year we went back to Carnac but stayed at a different site - we wanted to have a mobile home but couldn't afford the Grande Metairie's prices so went for a cheaper site instead. It was a good site, but lacked the facilities of Grande Metairie, I guess you get what you pay for!
All in all, an excellent site, one worth saving the pennies for!
Hope this helps!