Depends on what type of entertainment etc you want but if you want a quiet site, where your four year old is safe and can play on the swing,slide etc while you sit and watch from your van/tent and they can run around the 10 pitch site safely, following the chickens etc then Le Chant is good and has many excellent reviews. There is a small above ground pool which is great if hot. They also do pizza nights etc which you are free to join in or not....but great for parents as the children can join in and you can have a glass of wine while someone else cooks for you but without the expense of a restaurant or babysitter.
We went to Bel Ombrage : http://www.ukcampsite.co.uk/sites/reviews.asp?revid=6549 when our kids were 2,5 and 6 and it's a lovely site with littlies, there's a small stream that runs through the site that's great for kids to play in safely. See my review from 2006 for more info, others have been since and it's equally nice. It's Dordogne, so further than the Loire, you'd probably need an overnight stop around Orleans.
We travelled from Calais last year first to the Dordogne for a week (overnight stop is a must) and then back to the loire for 10 nights.
We were not over impressed with the site we stayed in the Dordogne (Lascaux Vacance Yelloh) but can highly recomment the site in the Loire - Parc de Monsabert. We have been a couple of times now and it is a lovely site - see my review.
------------- Plans 2012:
Hopefully buy a trailer tent and then maybe explore Burgandy!
Don't forget to leave a review of the French and other European campsites you have visited!
My family (wife and 3 kids - 5yr, 3yr, 1yr) stayed at Château de la Grenouillère in Suèvres for 2 years now. excellent for the kids, great pools and slides, playground, bike/buggy hire, good food/bar and shop plus lots to do around the area.
it takes about 5 hours from calais (without stops) and we have managed it fine (via calais - rouen - evreux - dreux - chartres)
try gien les bois du bardelet countryside but only 4 miles from the supermarket in the town made for kids - actively wants young families run by the daughter these days who is also a young mother therefore the interest lake indoor pool separate indoor area and pool for little ones you'll love it