Touring France you shouldn't have any problems finding a site - almost all french sites will take motorhomes. However, if there's four of you and three children, I presume you are either in a very big motorhome, or there are two! You might need to ring in advance if you want pitches together, or a very big pitch.
Don't know the web addresses for the following, but the Castels and Camping chain are good for kids in your age ranges, as is Yelloh, on Sunelia. These are all the bigger type sites - with plenty going on. Type into Google and see what you find. The campingcheque.co.uk website gives you an idea of what these bigger type sites are like - try them for an initial look at the facilities available. If you're looking for smaller sites, almost every town has a municipal site, some of these are fantastic. Try the French Tourist office for information.
Sites in France are much more widely available, and outside the peak season (mid-July to mid-August) usually you can turn up as long as you don't leave it too late (however, see above if you've a very big outfit, or want adjacent pitches), and find a pitch. Pre-booking is not always necessary - so you can make your itinerary as flexible as you like. If the weather is bad, move on, if it's good stay....
There is another website which gives information about 'businesses' which encourage motorhomes to stop over - restaurants, and the like sometimes offer free pitches if you eat there. Often french villages have a motorhome point in the square with electricity on a meter - the one in Sauze Vaussais is right outside my friend's house, so she has made friends with lots of motorhomers! If it's the first time you've been you won't believe how 'camper friendly' France is. On some parts of the mediterranean coast, for example, specific areas of the 'prom' are set aside for motorhomes to park up for a night. They know that we caravanners/motorhomers bring much needed business into their towns and villages, so we are encouraged. Motorway service areas are provided with specialised areas, and don't charge you, like in this country, if you overstay your '2 hours free'.