Where in the uk are you travelling from? The other channel routes to the west may save you driving on this side and on the French side plus the trip around the M25. Do your sums what may seem an expensive crossing is balanced out with the price of fuel, child hassle factor (are we there yet) and the zero chance to unwind on a short busy Dover crossing. The south coast of Brittany is an option and the northern part of the Loire-Atlantique coast line is another option, generally further south better the weather but it's not the Med. As it's your first trip may I suggest using the Caravan Club the sites shown cater for all tastes and the full booking can be sorted out with one phone call.
Tips, try not to drive to far in a day, its ok people telling what is achievable, this is you first trip and its up to you and your party what you think is right. Even though its 7-9 days try a 2 centre, its better having 2 / 3 nights on a site within a couple of hours of the boat, than a long drive to catch a ferry. Also that's a good time to stock up with items to bring back what's the point of loading up and lugging all that weight back to the coast when it's more or less already there.
Equipment, try and get hold of a reversed plug for the inevitable reversed polarity with the electric. Gas I have always took GAZ to Europe you can take much smaller bottle, its easily exchanged at local shops and as you are unlikely to meet with on or below freezing temperatures there really is no need for propane.
Food, if children are on the small side just take there favorite cereals, milk most is UHT but you can buy fresh milk at the supermarkets, you just have to look. When my to were young the quickly decided that chocolate quasants were more to their liking. I have seen people on caravan sites with their vans full of English food, looking more like a mobile shop than a caravan. The large French supermarkets make your average UK one look like a corner shop. The restaurant / cafeterias such as Flunch, Casino give good value and are child friendly. Also just pack what we call an 'emergency meal' based around dry food, just in case.
Fuel, most large supermarkets have fuel, just watch for height access restrictions though, if that the case and you are with the van, just un hitch leave the van with the wife in the carpark fill up and return.