If you want a French campsite, but with loads of other English, then get the Eurocamp, Canvas Holidays, and Keycamp and any other 'tour operators' (Thompson now offer campsites) brochures, and choose a site which features in most of them. This way you're likely to be at a site with other English families. You can choose your site - and then if you're taking a Caravan, book directly, or through the Caravan Club, or Caravan and Camping Club. Alan Rogers also offers a booking service. Sometimes the clubs can be cheaper, particularly if you book an 'independent' package, ie just ferry and site, rather than their all-in service, with maps, guides, etc. If you want a mobile, then it's always worth checking with the site to see if they offer their own mobiles, as sometimes this can be cheaper than booking through a tour operator.
You may find that your teenagers prefer other nationalities when they get there! The 'exotic' often attracts. I know both my boys made friends with girls from Holland, France and Germany when they used to come with us!