For WW2 history, without going as far as Ypres or Arras, there's the coupole near Saint Omer and Eperleques bunker. Cap Blanc Nez or Gris Nez for great views on a clear day. Nausicaa aquarium in Boulogne.
Google tells me it's an hour to 1.5 hours driving from Calais to Ypres.
The war was fought all over that part of France and Belgium, so wherever you go you'll probably find a cemetery to pay your respects. How old are your dc? If they've been doing ww1 at school, it would be worthwhile for them to visit a cemetery, I think. Needn't take up the whole day, so wouldn't be too oppressive.