In reply to rockrat:
What we've always done (from Cockermouth, so almost identical journey) is drive down late one evening to Travelodge on far side of London, such as by Dartford crossings. Leaving Cumbria at say 6pm gets you there before midnight, so a reasonable amount of sleep. And then get a fairly early ferry (for us, ferry works well, as it's an hour or more when they can run around fairly freely), get in at lunchtime, and then it's about 4-5 hours on the other side, so at campsite in time to pitch tent and cook tea.
We've tried a couple of campsites over the years, but La Musardiere near Milly la Foret is now winning because it's opened its swimming pools and they're ace for kids. Also an expanding playground. We've not found it noisy in evening - if you don't want electric hook ups, camping goes quite a way back into the woods away from the busy areas.
I have a list of good areas to take kids, at least at the pottering end of things - ie those that are either nice and flat and/or have kiddy white circuit, and have yellow/orange circuit for the grown ups. I'll try and dig it out, but hassle me by email if I forget! The Elephant, and Diplodocus are two obvious, well known ones that have kiddy friendly areas to play - although not necessarily right by the bouldering - we've tended to end up with one adult pottering off to boulder alone unless we've managed to rope extra grown ups in. But that's also because we're both really trad climbers, so going round a long but fairly straightforward circuit has as much appeal as working individual hard problems.
We have the purple Fontainebleau Climbs guide, which has a symbol to tell you if areas are kiddy friendly or not.
Oh, and consider an in car DVD player
