How about Germany - lovely lakes, fairy tales castles and immaculate campsites. You could make the trip an adventure too - ferry or tunnel then into Holland and Germany. Dutch/German border is about 3 hours max by car from the ferry ports. Stop overnight somewhere around Aachen/Duren or perhaps and head South past Brussels and Liege (a nice place in itself) and get to the Eifel National Park (http://www.nationalpark-eifel.de/go/eifel/english/Getting_here__und__accommodation/Getting_here__und__accommodation.html). Fantastic place with lots to do for kids (and lots of other kids too). Guaranteed a tired child at the end of the day and usually tired (but happy) parents...
If you can tear yourselves away from Eifel another couple of hours drive will put you in Koblenz on the Rhein. Head South along the river and you'll see fantastic castles (and just happen to be in wine and brandy producing country too). There's castles all along the river (http://www.loreley-info.com/eng/rhein-rhine/castles.php)). I would recommend sailing on the rive though apart from being fairly fast flowing it's very busy. For that head up towards one of the big lakes in Germany - Mohnesee would be my choice - there's even an RYA sailing school there - that'll give less than 4 hours back to the ferry ports.
Just to let you know, for all the reading and planning I did...
We have gone sailing, we have been out on the Dinghy and are now on the big boat. The forecast as it is, I/ we decided that it would be cooler at sea...
Presently berthed in Poole. Plan had been Swanage bit of thunder lightening which followed/ brought on Sea sickness meant an expensive but quite night was required.
We will still go camping later in the week. When compulsory attendance to a party is required.
How is it that just because you are sailing with nothing booked some none sailing family decide. You can come back early from your holiday...
For future ref: the lakes south of Arcachon near Bordeaux are worth a look. Local friend has camped twice at Biscarrosse at a lake side campsite with his boat on a jetty. Longish haul from the Channel but good weather. Wine Chateaux around the Medoc area. Climbing, well, sort of, on the Dune de Pyla. Last few days here over 30C. Less chance of mal-de-mer on the lakes.