My vote is to go for it, too!
We go to WDW 1 or 2 times a year, and DL between 4 and 8 times a year. They are both awesome destinations with plenty to offer a growing family. With your kids in the 11 & 14 age range, they'll have the stamina and the intellect to absorb all that WDW has to offer.
We go to WDW in early December, and the weather is very unpredictable. Living in Vegas, we RARELY felt it was warm enough to swim, and usually need sweatshirts in the evenings. It has been cool enough to warrant hats and gloves---just like January at DL this year!
I agree with the suggestions of staying onsite. I think it dramatically minimizes the planning stress. Free transport from the airport means no rental car and no hotel commuting hassles. I don't know your budget or habits, but if this is a "once in a lifetime", I'd suggest at least a moderate resort. Ideally, I'd try to rent points from a DVCer, but early December is a very busy DVC time, even though it's lowest crowd season in the parks.
The suggestion for the dining plan is interesting. However, IME the dining plan eats a lot of park touring hours, because you are having sit-down meals every day. The meals are great, and we enjoy the dining plan, but unless you are a foodie, you might want to skip the plan, spring for a few choice meals OOP, and work the shorter park hours as best you can.
Wow---enough of my rant! Read HydroGuy---he's got great information. I'll sit back down in my seat until I'm called on.
