Hi Jodie,
How exciting to be planning your first trip to Walt Disney World!
I suggest that you start by taking advantage of some of the resources available for planning. You might start by watching the
Walt Disney World planning video, so that you can see many of the attractions at the theme parks. Also, peruse the
Walt Disney World with little ones website. Especially since your girls are so young, you can find great thoughts on planning for those littlest children on that website. Also, this
Disney Parks Blog post on height requirements for attractions might be of help to you. Especially your youngest child, and even the older ones, might not meet the height requirements for some attractions.
I am so glad to hear that you are thinking about how to use
Disney FastPass+ service for your vacation. As you know, this will allow you to experience some attractions with shortened wait times during your visit. I like to make selections for popular attractions that will likely build longer queues.
While I am limited in space and will not have enough room to cover all four theme parks, let me just make some recommendations for Magic Kingdom. Check the height requirements and then consider Space Mountain, Splash Mountain, Big Thunder Mountain Railroad, and Seven Dwarfs Mine Train. These rides are very popular - and quite fun! For those without height requirements, there are some popular attractions in Fantasyland, such as Peter Pan's Flight and Dumbo the Flying Elephant, for which I like to have Disney FastPass+ selections.
Please do let us know if we can help you with any other questions, Jodie!