I saw several landyachts keel over because they didn't look out or couldn't avoid them at high speed. When the tide is comming in this can become dangerous. Sometimes the sandbanks are soft and slow you down to zero. Even with midi wheels. Don't forget your bicycle.
Campings are always more in-land then close to the beach.
Just buggy during the day. They are there, for free, on the beach, at low tide. But stay away from the official growing fields. Berck sur Mer, Le Touquet. Check Decathlon in Lille for a cheap kite.