DFDS Dover-Dunkerque is the cheapest ferry but obviously more direct ferries will save you time, don't know anything about them though. You are going in low season so get an ACSI card then the most you will pay for a site is €16. Have a look at my reviews of L'Ptit Bois near St. Malo, or if you want to go further west, Les Plages de Beg Leguer near Lannion. Both lovely areas, if you do decide to go, I hope you have better weather than we did!
It's a low season discount card which is accepted any many sites abroad (and some in the UK), normally from site opening day until the end of June and then again from the beginning of September until site closing day. Some sites accept them at other times though. Look them up on Google. You pay at the site rather than in advance. Some sites do deals such as 7 nights for the price 6, or even better if you stay for longer.
Vicarious Books sells the books of participating sites and related info and the card is attached to one of these 2 books (so many sites now that 2 books are needed). A few sites only charge €10 per night, not that we found or used any of these.
You can also buy Camping Cheques, often as part of a ferry deal, cost £13.95 each and you pay in advance. Some ferry deals will give a few free cheques. Lots of sites also accept these, but not as many as ACSI.
Don't forget to leave a review of the French and other European campsites you have visited!
I doubt you can ever guarantee will be child free, unless you go to an adults only site. Sadly dont know any of those, but in September you are likely to have little kids, but the bigger ones should have gone back to school.
The area around Carnac, is great for older couples, lots o history, sightseeing and lovely warm sheltered coves.
ACSI Card website is www.campingcard.co.uk - and despite Warbis' warning about children, you really rarely see them on French campsites outside school holidays. There might be the odd Dutch couple with a baby or toddler, but few others.
We holiday every year at that time, and don't even think about 'choosing' a site - we go where the weather is best and find a site in that area - sometimes this is inland, because there are many beautiful lake and riverside campsites in France, as well as those by the sea. We buy a paper when we arrive in France and head in the direction of the best four day weather forecast, stay whilst the weather is good and we're enjoying ourselves, and move on if things change.
You really don't need to book and Europe is your Oyster!
Don't forget to leave a review of the French and other European campsites you have visited!