What did you decide in the end? We're thinking of doing a similar thing this summer, though still torn between a hovercraft to the Isle of Wight, and eating Crepes in France
The Isle of Wight has so much for children to do http://www.dayoutwiththekids.co.uk/search.php?county=Isle-of-Wight
The only benefit of Calais/Dieppe is if they're learning french, or if they want to get a stamp in their passport