The Camping France website lists ALL the official campsites in France, but gives only brief details - though web links are there. http://www.campingfrance.com/
If you want more details then try Alan Rogers - who do a guide which reviews campsites. http://www.alanrogers.com/?gclid=COLvo6rShJ0CFaAA4wodJ1wpaw
If you want a larger site, with other British families then try the Eurocamp, Keycamp or Canvas Holidays websites which give details of sites where they have tents. If there's anything you fancy then you can look for the site's own website.
You can have a look at the Camping Cheques (www.campingcheque.co.uk) website - although Camping Cheques aren't accepted in many sites in July it can give you brief details of each site in France, and then links to the website of the site itself.