Why not leave the car behind and take a day trip to Calais by ferry as a foot passenger!
There is a shuttle bus from the ferry port to Calais town centre where you can find lots of cafés/restaurants to feed your hungry children and get some idea of how they wanted to spend the day. This advice comes from a France For Families visitor who lunched in the Café de Paris and reports that it is OK….but not the best. After lunch they walked to the beach (about 25 mins) where the kids had fun in the sun and water and passed time watching the local children turn somersaults in the sand. If you’re shopaholics then there’s plenty of shops to browse in the town centre.
Apparently getting back to the port is troublesome as pedestrians, unfamiliar with the area and no knowledge of the language ( except for the odd word which English has an association/similarity). We would recommend buying a map (une carte) when you arrive and to carefully work out your return route earlier in the day. If you can take a taxi this might be the most straightforward.
This from a France For Families’ reader: Transport between port and town is not as difficult as suggested above. There is a regular bus service for ferry passengers that does a circuit from the port through town with clearly marked stops on main street and at railway station. Cost £2 or 2 Euros per person – pay driver in pounds or euros. Timetables clearly displayed on stops. We usually use the stop near the tower as it’s midway between beach and main town. Just make sure you note the time of your return journey as they are not very frequent (hourly?) and if you miss the bus you’ll need to get a taxi.
Find out more about Day Trips in France: