The whole coast from Calais to Le Crotoy has lovely sandy beaches.
How old are your DCs? I'm sure you'll find something to do.
There's a maize maze in the summer months inland from Le Touquet and also a tree top adventure thing, Azincourt battle field, Montreuil sur Mer historic walled town, Le Touquet for posh (window)shopping, the Marquenterre nature reserve, WW history sites,
For wet days, indoor fun pools in Boulogne and Le Touquet, ice skating rink in Boulogne.
You could cross into Belgium. Sunparks (CP but cheaper) have a couple of parks on the coast. There is a tram that runs the entire length of the coast so easy to visit the different towns. There are theme parks/aquariums, lovely sandy beaches and some good restaurants.