There are so many excellent campsites in Brittany, it's difficult to pick one. I would suggest browsing the Eurocamp or Campotel brochures for ideas. Then contact the site direct, they all speak excellent English, and negotiate a price. You will save hundreds but check that your kids can avail of the full range of activities. Roscoff is a slightly shorter crossing and is more convenient to the west coast. If you are prepared to drive a few hours more, the weather in the Vendee is more reliable.