Hi and welcome!
I would recommend sticking to WDW for this trip. On our most recent trip to WDW, we had seven full park days and still didn't see or do everything that we wanted to in WDW.
Epcot has tons to do for kids that age! We spent 1 day each at Hollywood Studios and Animal Kingdom and the rest of the trip was split between MK and Epcot. MK is the park you will most likely spend most of your time in but Epcot is second for us. My DD was a little over 4 on our last trip and loves Epcot!
First, there are lots of rides and attractions for little ones - The Nemo ride and Turtle talk with Crush, all of the activities in Innoventions, Living with the Land, Journey into Imagination, your 5 year old should be tall enough for Soarin and Test Track.
My DD loved the Kidcot stops which are in Futureworld and World Showcase and they are free! We also bought a passport (around $11) which she got signed and stamped in each country.
Another great thing a lot of people don't realize about Epcot is that there are tons of opportunities to meet characters there and the lines are usually a bit shorter than in MK. My DD loves meeting the characters and getting their autographs - she loved this as well on our first trip at age 3 1/2. Just a few of the characters we met in Epcot are Jasmine, Aladdin and Genie, Marie from the Aristocats, Mary Poppins, Mulan and Mushu, Pooh and Tigger, Alice, etc.
Have fun planning your first trip! We loved our first family trip so much that we went again 9 months later!