Try Amboise. It's about 4 hours from Calais (that's with a car and not a speed-restricted caravan). You can walk into town, there's a small cafe and the municipal pool is next door.
Thanks for the suggestion and yes we will be travelling by car. Amboise looks beautiful although we haven't considered municipal sites yet for the moment we are looking at 4 star sites with amenities to keep our children entertained.
You might like the four star site Camping Le Champ Neuf on the Baie de Somme. It gets good reviews on here, and other review sites. It's near to several pretty towns and villages to visit, and close to some historical sites. Nearby are lots of long flat beaches, and the nature reserve of La Marquenterre which has lots of storks nesting on tree tops and you can take some great photographs of these and other birds such as spoonbills, glossy ibis and many others.
The site has an indoor pool just in case of poor weather and pitches are generous. You can order bread, there is a 'Friterie' and a pizza wagon visits in high season. Toilet blocks don't have toilet seats - another common thing in France but unnerving for a first-time visit for some people.
You will need lycra type swimming shorts - like many other French sites this is one of the rules which is enforced.
We stay in Saumur at Le Camping Ile D'Offard. locataed on an island right in the middle of the Loire. A great site for kids, good menu, relaxed pool and good service at the bar.