Where to go depends on what you like. I think I'd start by looking at the Eurocamp/Canvas Holidays/Keycamp brochures. This gives you a feeling for the types of site which might suit your children - with often a children's club, etc. However, if you like very quiet sites, like CLs in England, then Camping a la Ferme, or Aire Naturelle, may suite better. When you've picked out the sites which appeal to you have a look on their websites for prices, etc., or if you're organising your trip through the Caravan Club or Caravan and Camping Clubs, then phone them and ask for advice/suggestions/brochures about timing, ferries, etc. I suppose you're limited to school holidays, so a word of advice is that sites are much less crowded at the end of the British school holiday (last two weeks in August) than at the beginning in mid/late July. There are still plenty of people around, but traffic on the roads in less, too. All the bigger sites will still have their full programme of activities on offer, so your kids shouldn't lose out. Ferry crossing is also important - when you've chosen a site, work out which ferry crossing suits you best. Dover/Calais or Dover/Dunkerque with Norfolk Lines are often the cheapest - the longer western channel crossings are much more expensive. If you're doing it for the first time there's a good article on another website (UK campsite) for novices which would be worth reading. Eurocamp Independent or Select Sites can book you a 'package' including ferry, site, and route maps, of Eurocamp offer a 'courier' service on site which can help if you have any problems.
There are some nice sites in northern France, but you don't get the weather - although it will give you a taster of the french experience. Some of the Normandy sites are great for first-timers, and there's loads of lovely beaches in the area.
Don't worry - we all did it for the first time once, and some of us (Us!!) have carried on for the last 26 years or so!!!