Hi Rebecca! There is so much magic in every Disney park at Walt Disney World I am confident your children will be excited even beyond the great Mickey Mouse and all the beautiful princesses!
You might consider letting your girls become princesses at the Bibbidi Bobbidi Boutique. They can then enjoy the Magic Kingdom dressed as their favorite storybook character. The two top dining with princess choices are Cinderella's Royal Table in the castle of the Magic Kingdom and the Princess breakfast in Norway in Epcot. Check with the Guest Relations at each park to see where and when the princess meet and greets will be in each park on the day of your visit.
Your youngest will find Mickey Mouse in all parks and top meet and greets include the Town Hall in the Magic Kingdom next to Tony's restaurant and the character spot in Future World of Epcot where you will also see Goofy, Minnie, Donald, and perhaps Pluto. Make sure you stop at Disney Junior in Hollywood Studios for a great live show with the Disney TV favorites.
I don't have room to list all the must do's for each park for each age but Disney has compiled a favorite list for age groups for all attractions that you should review. Click
HERE and
HERE to see their suggestions. It is broken up into Pre School and Kids categories. Let me know if I can then answer any specific questions you start to have while in the planning process.