I recently went down to Algeciras (nearby) from Salisbury via Dover-Calais/Paris/Northern Spain etc.
It took me three days on a motorbike and to be fair, the journey was fairly dull.
I had to get down to Algeciras to get over to Morocco, so couldn't use the back roads etc it was motorway all the way sticking to about 70 - 80mph.
I wouldn't want to do it over a shorter period and if you have plenty of stops, it shouldn't be too bad for the children over three days.
Make sure you have lots of games for them to play and are you prepared for the "are we there yet" comments every 30 minutes :-)