I never satyed in a camp site but my Fiance is from Fougeres and I stayed in Lorient in south Brittany for a year. In my experiance the weather in Brittany is a mixed bag. They have a saying that all the four seasons happen in one day. One minute the sun is shing next minute its downpour. It has the same kind of weather as Ireland constantly changing all the time. though when its warm its very warm but never expect it to last too long . I would bring raingear, sun clothes and the odd jumper in case it gets chilly!!!
Have a great time and eat out in a Creperie and try pommeau and Breton Kir!

many thanks for your comments - my wife has taken the info on board and has purchased an extra suitcase for even more clothes!!! - can't wait to try the local cuisine and plonk

You ll be fine!! Have a great time!! And just one final thing it could be very hot in August. You never know for certain so don't forget the sun cream especially for the kids as even I was burnt in April last year
Good luck