naomi; Dunkerque interesting!!! I wouldn't make that suggestion to my worst enemy.
keyplus; Prior to your visit make sure your children know the historical facts behind the story of the Burghers of Calais, it is a great 'existentialist' story. Then take them to see Rodin's magnificent sculpture outside the ugly Town Hall. I know there is a copy by the House of Parliament but this is where it was created for. Try to enthuse them to make some simple drawings from parts of it- trying to draw it all would be far, far, too demanding. This is one of the world's greatest sculptures, and they will remember it for the rest of their lives.
There are some good restaurants across the bridge, but it isn't as nice as old Boulogne.