Hi Daphne!
I would head to Epcot on arrival day and save Hollywood Studios for a "whole" day. With your little ones, I think you'll find Epcot on your arrival day to be a fun place to explore and have some great food, watch Illuminations, and then head in "early" (compared to an evening EMH). You'll still be able to get a full night's rest and then make a whole day out of the park you choose to visit the next morning.
My young kids really enjoy Future World in Epcot.
Innoventions (
East and West) and Imagination! are their favorite areas to play at. These areas are indoors and very interactive. Even my little one is able to have fun while he explores alongside his big brother. You should find you are able to explore these areas in half a day.
Hollywood Studios also offers many fun attractions for the little ones, and while it is an easier park to tackle in one day, this will actually allow you to enjoy all those experiences at a reasonable pace.
Don't forget to check out the park maps highlighting many of the great attractions geared towards little ones. Look for the pdf
here on the right menu under "Magical Beginnings". It's a great resource!
Hope this helps! Have a magical vacation!