Hi Emily,
Welcome to the Disney Parks Moms Panel! How exciting to plan a trip to the Walt Disney World Resort for your girls!
Love your strategy - taking your time in the parks will allow you to enjoy more attractions, especially with little ones. Though you can certainly find enough to do in Magic Kingdom for three days and Epcot for two, with the ages of your girls, I would definitely visit Disney's Animal Kingdom. When my girls were younger, they stared in wonder at the animals and loved any moment that an animal specialist told them some interesting fact. They especially loved The Festival of the Lion King show. It is such a beautiful park with gorgeous plant life. We find it quite relaxing actually. When our girls were about the same age as yours, they loved to take a break at
The Boneyard and we loved letting them just play for a few minutes. If I were you, I would consider doing two days at Magic Kingdom, two days at Epcot and one day at Disney's Animal Kingdom.
The great news is that with multiple days in each park, you can spread out your Fast Pass+ selections and make them for three rides per day that you have valid theme park admission. We always spend two days in Magic Kingdom and love being able to make more Fast Pass+ selections in that park. For your girls, you could make Fast Pass+ selections for The Many Adventures of Winnie the Pooh, Dumbo the Flying Elephant and Astro Orbiter one day then "it's a small world," Peter Pan's Flight and The Barnstormer on another day. Remember that once you have used your initial three Fast Pass+ selections or the ride time windows have expired, you can make a fourth, "rolling" Fast Pass+ using the My Disney Experience Mobile App. Also, be sure to check the height requirements.
Emily, I am sending pixie dust your way as you plan!
Magic, memories and Minnie Mouse hugs,
Donyell