I would highly recommend Le Clos Cacheleux. It's not a farm stay but it is a very nice rural site with animals, just over an hour from Calais. Very quiet site when we were there with large, pitches, more amenities (pool, inflatable etc) at sister site Le Val de Trie, 10 mins walk with small children, 5 mins without! We went in early June with children the same age and it made our journey home a break in itself. Le Clos Cacheleux
To be honest, and I know this might go against the grain here, I was thinking of not camping, I should have made that clear in my post.
We're leaving Carnac in the morning and breaking camp for us, with the children, takes a couple of hours so I was thinking of staying in a bricks and mortar place near Calais for that night and the next, to avoid putting the tent up again.
Don't suppose you know of anywhere near the site you have stayed at?
I've looked at the Gite de France website but finding it difficult to use!
What about this one in Rang du Fliers - trouble is it doesn't have any availability in August but might just be worth an email to check as their website was last updated in April 2012.
Try googling in French 'chambres d'hote a la ferme, pas de calais - and you'll get more results. I've just been looking through a whole list of about twenty farms offering bed and breakfast in that area - but the websites are in French. Use google translate if you need a translation of the web pages.