We've been to Center Parcs twice and loved it. If you go on their website they sometimes do special deals and you get a discount if you're taking a pre - schooler. My only advice would be to book up the activities you want to do as soon as you can to make sure you get the times and days you want.
We also took our two ( who turn 5 next week ) to EuroDisney for 3 nights in October and as Dot said that is the perfect length of time. I think the flight to France is also just the right length for a four year old -by the time they have a snack, go to the toilet and look at the clouds for a while, they're there. Word of warning - the price of food at eurodisney is ridiculous. We ended up smuggling croissants and stuff out from the hotel's buffet breakfast!.