We went to the het heijderbos centre parc in holland a couple of years ago.
We did Dover to calais, (we live about an hour the other side of london) then drove down to Ypres (Iepers), took just over an hour and stayed there for the night - we got a good deal (£70/night for family room) on a Novotel that was just off the main central square and included parking (although we did buy breakfast from the very nice bakery over the road rather than pay for expensive in-hotel breakfast).
We then drove to het heijderbos the next day - about 4 hours away, having had a wander around the town first and having had an explore the afternoon/evening before.
Coming back we stayed in the same place - the dc were 3&6 so liked going back to a hotel they had already been to. Ypres is a lovely town to wander around, lots of it rebuilt in traditional style after the wars. Also amazing war memorial stuff. Lots of tourists go there, which means that for a town of its size there's quite a lot of restaurants and things going on. We were able to have a wander again in the evening before eating, then the next morning we had a quick walk before shooting up to Calais, where we went to the massive shopping centre to stock up on things from the hypermarket and nice French shops. We also bought stuff to have a picnic whilst we were parked up and waiting for the ferry - I think we had a nice snack at the centre (good ice cream, waffles and crepes!) so we could have a late lunch. That worked well as it meant we had something to do in the dead time enforced on you between turning up in good time for the ferry and actually being loaded on board.
It was a lot of driving but we figured we would enjoy the journey rather than have a nightmare drive of everything squashed together.
We've done similar things going to other European destinations - we've found that Novotel/ibis hotels are good for family rooms, as are holiday inn express (who include free breakfast buffet in their price, and we've always had really spacious rooms). It's worth signing up for their loyalty clubs; doesn't cost anything and often have good deals - half price if you hit it right! You just get occasional emails telling you the latest offers.
Depending on where and when we've been and other commitments, we sometimes stay in the same place on the way there and back, other times we will stay in different places. Sometimes it will be for a night, others it will be for two or three nights. But we've found that for us, aiming for a lunchtime ferry crossing and then an hour - hour and a half's drive beyond Calais in the right direction works really well. You get to the hotel with enough time to explore the town in the late afternoon and evening, if the hotel has a pool then it's great to relax and cool down before eating. Next morning a brisk walk to wear out the dc before the car trip so they're not too fidgety and you can still get to your destination by early afternoon for a late lunch.
Hope you have a lovely time on your holiday!