We stopped in Ypres (Iepers) in Belgium when en route to Holland with the dc a couple of years ago. It was a lovely town, small enough to wander around in but loads of history to look at too. Maybe an hour and a quarter from leaving the ferry.
Alternatively, Bruges is nice and different although touristy.
Actually - Lots of Belgium would work as it's all in reasonable driving distance of Calais - Brussels is nice too, and it has great selling points for kids (like chocolate. and chips with mayonnaise) and adults (lots of beers in lots of different glasses) plus plenty of stuff that's not too outdoor/beachy based if the weather does turn bad.
Centre Parcs in Belguim and Holland are much cheaper than in the UK although I'm guessing they will probably be in their peak time around Easter so not as cheap as they can be). Actually I've just checked CP in Belgium for a mid week break monday - friday leaving on the good friday and it's 239 euros for a 3 bed cottage that sleeps 5 or 10 euros less for a 2 bed cottage that sleeps 4, which seems pretty reasonable.
There are also other one off places - Duinrell is one that I've wondered about doing, cabins to stay in, with a funfair, rides and swimming type complex you get access to.
Alternatively if you want to stay in France, Le Touquet is nice and there was a nice hotel on the beach that had a great indoor pool that overlooked the beach and sea (great if weather isn't perfect!) as well as a spa for you 
Plus it wasn't a massive town, so nice to wander around with younger dc.
We've also used Arras as a stop off point within easy striking distance of Calais - we used a Holiday Inn Express that had a huge family room and included a delicious big continental breakfast buffet in the price (lots of hotels seem to make a big profit on breakfast!) for a very reasonable price. I signed up for their rewards card which meant I could book the room at half price so worth checking for deals like that!