Depends on how far you want to travel but La Rive in Biscarosse is on a huge lake with it's own lake beach and a 10-15 min drive to the coast it is a big site and has a big water complex which includes an indoor pool as well plus lots of other facilities. As said it is a big site which does get busy in high season but lots going on for the children.
Not sure why all the shouty exclamation marks as you didn't mention any location in original post. Sequoia parc has a good water park and is very close to Marenne lagoon beach. Pitches are generally huge too we had a very sunny good time there a few years ago.
Don't forget to leave a review of the French and other European campsites you have visited!
Les Brunelles has an amazing water park and is in the Vendee area. It's near to the beach too (800 metres) and after the 24th August will take Camping Cheques, so if you can make that your second week you could save 22 euros per night between the 24th August and the 31st August.