Your first trip to WDW will be simply magical. The holiday festivities just add to the magic. The crowd level will depend on how close you are staying to the actual holiday. It will be very nice up until about December 21 and then I would expect the parks to become very busy.
I would order a free
Walt Disney World vacation planning DVD and share it with the kids. In each theme park you will find attractions for both of your children to enjoy, either separately or together. The attractions that you visit will depend on your child’s interests and tolerance for ride intensity.
Girls will find lots of princess experiences at the Magic Kingdom, along with some rides geared to the younger set. Have a mini-flashlight to carry while on “dark” rides such as Peter Pan. Your son can see Capt. Jack Sparrow’s Pirate Tutorial and take a pirate oath.
Consider having one character meal.
Your son may enjoy seeing the Power Rangers and Incredibles at Hollywood Studios, along with awesome shows such as Indiana Jones Epic Stunt Spectacular and Lights! Motors, Action! Extreme Stunt Show. The Jedi Training Academy is also lots of fun. See Voyage of the Little Mermaid for a great family show.
In Epcot, visit the Seas with Nemo and Friends. Both children should have fun exploring the exhibits as well as seeing Turtle Talk with Crush.
At Animal Kingdom, enjoy the Kilimanjaro Safari. Both children will enjoy the Boneyard playground.
There are many more attractions (rides and shows) in each park for everyone to enjoy.