Well, it's hard to say which park will be least crowded, or best to start out with, but I can tell you this -
Magic Kingdom is almost always going to be the most crowded park, especially by noon, the crowds will be peaked.
Animal Kingdom is likely to be least crowded, yet, they close at 5:00pm, I believe, and you really wont have enough time to get much done.
Epcot and MGM are always somewhere in the middle. Not empty, but usually not overly crowded either. Experience has shown me that Epcot's crowds, on any given day, are the most managable. Plus, Epcot has a number of great attractions your kids will probably want to get right into.
However, and this is purely personal preference, no matter how crowded Magic Kingdom may be, I dont consider myself to be at Disney World until I see the Castle. But, if that isnt an issue with your family, then head on over to Epcot, I think it'll be your best bet.
BT