Have only once stayed near Calais and that was for 1 night waiting for a Eurotunnel. The site was in Boulogne and bluntly it wwas b....y awful. I believe there are better sites a bit inland, but there are a number of fairly obvious reasons why I would give Calais itself a wide berth.
Personally I would head further south towards Abbeville and look for sites around the Bay de Somme, which is a very attractive area with a lot of sites and plenty to do. It is only 2 hours or so driving and far better area.