Can I suggest you specify which crossing you think you will use? Many people will have loads of suggestions for you and while the Bien assise is certainly near the port of Calais, if you are looking to get decent weather you may be advised to look at crossing to Brittany or Normandy.
Well Ive stayed at two sites in Pas de Calias Nord that I found on the ACSI website.
Le Perroquet in Bray Dunes: Hard to find cam psite right on the Beligan/French border. Small cramped pitches, hedged to make it as hard as possible to back in and set up! Near the beach but nothing else on site for the kids. V. Friendly staff, v. nice décolletage on the attractive and helpful reception staff (!) but toilet block leaves much to be desired - cold, dingy, no toilet paper, limited warm water. Cold water supply only for washing dishes. Friendliness of the site made up for much, but Id like nicer loos before I go back.
Restaurant next to the site smells of French (or Belgian?) bad drains and having had a look inside at the hygiene we left and put a tin of soup on in the van.
I also stayed at Camping les Dunes at Gravelines. Mainly holiday chalets but with some places. Pleasant enough site, really geared to permanent holiday homes so rather basic camping facilities compared to sites Ive stayed at in Holland and Germany. Chemical waste had no running water, for example. I didnt use the toilets etc so cant comment on these. Cheap site though. Campers not so happy as at Bray Dunes, with customary Gallic reserve for foreigners. Pleasant reception staff though, but dont ask for any help with your pitch. You are defintiely on your own, even if solo camping. Pitch cost me 6 Euros in June (!!)
For both of these you would need to use the beach for kids recreation if you go as a family.
Fine for an overnight transit waiting for your ferry, but on balance I dont think I'd particularly want a holiday on either of em.
SL
------------- Veni Vidi Velcro.... I came, I saw, I stuck around!
Don't forget to leave a review of the French and other European campsites you have visited!
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.