If you were going any other time than July, you could stay somewhere like Chateau du Gandspette, using Camping Cheques (£13.95 per night, including two adults, car and caravan, awning, electricity and one dog). July is peak period, though, and you'd have to pay full site prices which aren't cheap. There are other campsites around, though, which could be cheaper, but aside from Le Bien Assise at Guines, we haven't stayed on any of them.
Gandspette campsite has a pool, bar and restaurant, and if the weather is good it can be lovely. Very nearby is the Blockhaus at Eperlecques (where the V bombers were built and launched). Calais can be quite interesting on market day, and there are some cheap restaurants down by the quayside (though don't expect Gourmet Food - Le Coq d'Or is amongst the best). Then the nearest city is St Omer, with a good shopping area in the centre and market, plus the usual array of hypermarkets, the La Coupole bunker (WW2 historical museum), and the canals of the Marais. Cassels is a small hilltop town which makes a worthwhile short visit, Ypres is within a reasonable drive - and worth staying for the 8.00 pm ceremony at the Menin Gate, then there are the coastal area to the west of Calais, Wissant, Cap Gris Nez, Wimereux, and Boulogne is a nice little place. You're also within about an hour and a half of Bruges, and Lille - both definitely worth visiting, and I'm sure there are lots of other places.
It isn't an area of exciting tourist sites, but a couple could perhaps make up a reasonable itinerary and fit in as much or as little sight-seeing as you wanted. However, if you have children, you're perhaps looking for something more beach orientated and I'm afraid it's a long time since we had children with us!