Hi, Tammi!
I bet everyone in your family is getting really excited for your Walt Disney World vacation! I know that it is sometimes difficult to narrow down what you want to experience at Walt Disney World Resort because there is just so much to see and do.
Epcot and
Magic Kingdom Park are both magical in their own unique ways. If you have the Park Hopper option added to your tickets, I would visit both parks on your third day. If you will not be park hopping, the decision may be tough to make, but I truly believe you can't go wrong either way.
I can tell you that my family usually visits Magic Kingdom Park twice on each of our Walt Disney World vacations. So many of our must-do attractions are found at Magic Kingdom Park that we like to have the extra time there. That being said, we also love Epcot. Many people have the impression that Epcot is more for adults, but that is simply not true. There are plenty of attractions in Epcot that are popular with little ones, and my own children really enjoy spending time there.
In addition to the Character Greeting with Elsa and Anna and the enchanting Frozen Ever After attraction, young guests will love The Seas with Nemo & Friends and Journey Into Imagination with Figment. Also, Agent P's World Showcase Adventure and the Kidcot Fun Stops are great ways to keep your children entertained while you explore World Showcase.
If you decide not to visit Epcot, be sure to catch the Festival of Fantasy Parade in Magic Kingdom Park. Your girls will be able to wave to Elsa and Anna as they make their way down the parade route.
I know whichever park you choose, your day will be magical!
Samantha