Your little ones are at a great age to take away a ton of magic from your upcoming Disney trip, Nora. My biggest piece of advice to you would be to allow them plenty of time for napping. When my kids were that young I found it was the only way to ensure we could all happily get through our vacation without any breakdowns. Also, do your best to keep them on a fairly familiar schedule with regard to meals and sleeping. It will do wonders for your trip!
As for which parks to visit, I can honestly tell you that each of the four major theme parks offers a little something for every age group, toddlers and preschoolers included. If it's possible, I would recommend visiting all of them. They might especially like
The Boneyard at Animal Kingdom and
Turtle Talk with Crush at Epcot.
If you plan to enjoy all of your meals on Disney property, it might be a good idea to add a meal plan to your vacation package. It can equate to a significant savings over paying out of pocket for all those meals, assuming you would order a beverage and dessert at each meal. Otherwise, you might see more of savings by paying out of pocket. Cinderella's Royal Table offers a truly memorable experience for princess lovers. If you can get a reservation, I suspect you would make your two daughters very happy! I hope that helps!