Lee - if you've got a child, then it's probably important to you that there are other children around for him to make friends with. Check out the Eurocamp and Canvas Holidays websites (www.eurocamp.co.uk and www.canvasholidays.co.uk). Both these companies offer self-drive holidays with mobile homes, but the sort of sites they offer have lots of other children around. There are several in and around the Loire area - which is within your 300 miles.
However, if you want beaches then 300 miles will take you to Normandy/Brittany border - there are some lovely sites there, too. Le Cormoran on the Cotentin peninsula gets good reviews from families with children, and is near the Normandy landing beaches which may interest you. Le Grand Large, also on the Cotentin peninsula has direct beach access.
But, reading your post again, are you just wanting an overnight site about 300 miles from Calais, on your way further south? If so, I'm sure lots of people could recommend overnight sites - especially if you let us know where your 'end' destination is.