We stayed at Chataeu Drancourt last year, at Easter.
Don't worry re French, the Keycamp staff were English and extremely friendly and helpful.
We were in a mobile home, but the facilities for campers looked excellent - good shower blocks, laundry etc.
There were several small play areas dotted around the site, plus a huge field for games, and lots of kids cycling and scootering around the site. Also tiny ponies tethered on the lawn to pat.
There is a big supermarket at the roundabout by the turn off to the site, Carrefour down in St Valery, and a limited, rather expensive shop on site.
The nearest market town - 4k away, St Valery, is lovely, stroll along the estuary wall at dusk, explore the little streets, and definitley check the market. Your children will enjoy the Chemin de Feu (steam train) which goes all round th bay to Le Crotoys on the other side. Explore the nearby seaside resorts and dune areas, Le Crotoys is a good 'bucket and spade' place.
I would guess the pool area gets v busy.
There is a good tourist information centre in the centre of St Valery with details of lots of days out etc.
Take loads of money - the Euro is expensive! But the supermarket has good deals on many foods - packs of 1E ready prepared salads etc.