I would definitely recommend Canvas Holidays (
http://www.canvasholidays.co.uk). Their prices are very reasonable and they have lovely locations all over Europe - we've stayed at the one near Venice, one in the Ardeche and their site near Paris (brilliant for daytrips to Disneyland). Facilities (including loos!) are excellent, and there's loads for kids to do. Our three quickly made lots of friends. Babysitting and kids' clubs are on site, too. We usually go with a group of friends and their children and everyone has a fab time.