Hi Nancy,
You have chosen a great time to travel to Walt Disney World. The weather is typically quite nice and mild, the crowds will be very manageable and the amazing holiday decorations should be up! Typically, those decorations are up in Magic Kingdom by the first two weeks of November in time for the first date of Mickey's Very Merry Christmas Party. So, you should be there when the parks and resorts are full of festive spirit.
Having traveled with toddlers there many time, I would most certainly concentrate your time in Magic Kingdom and Disney's Animal Kingdom. This is not to say that they won't enjoy parts of Epcot or Disney's Hollywood Studios. Be sure to head straight for Fantasyland in Magic Kingdom, as this will be where most rides and attractions are geared toward the little ones. Be sure not to miss the Festival of Fantasy Parade and meeting characters, too. My kids have always loved Rafiki's Planet Watch and the Affection Section at Disney's Animal Kingdom. Over at Epcot, there are
Kidcot Fun Stops which provide an opportunity for the kids to do a small craft in each of the countries of World Showcase. Disney Junior - Live on Stage is always a hit with my kids at Disney's Hollywood Studios. For more information, a downloadable parents' guide and a list of rides and attractions most geared toward that age group, check out this
official page for traveling with preschoolers at Walt Disney World.
If you use the Magical Express from Orlando International Airport to a Walt Disney World Resort hotel, you will not need a carseat. However, if you plan to ride in a cab or rent a car, you would need one.
Have a magical trip!
~Allison L