Keycamp (and the like) are very good, but they will still cost you a fair bit in the summer holidays. It will cost about £1000 for 12 nights in a tent, with the basic Dover Calais crossing mid week. A caravan will start at about £1200, 1300 for the same.
We have been to sites in Britany and Normandy and the Vendee and all were great, as long as you get the sun. And the kids clubs are fun.
Hope that you get things sorted