Hi, April!
You are in for a wonderful trip!
At your children's ages, my little ones would have preferred to spend two days in Animal Kingdom, simply because they loved the animals as well as dinosaurs.
Check out the Attractions available at Disney's Animal Kingdom Park so that you can see how many animal-based activities there are in AK, as well as the many kid-friendly shows you'll find there.
On the other hand, my kids loved Epcot as well. It's a much bigger area to cover, however, and their little legs could easily get tired before you've seen more than 1/2 of it. This is one reason why you
might want to consider spending a second day at Epcot, instead of AK. Check out the
list of attractions available at Epcot, paying close attention to the interactive component of many of them; there are many!
A compromise might be just what you need, however. If you have Magic Your Way Tickets with the Park Hopper Option, you won't have to choose between the two. We've often spent a second morning at AK, just to take one more safari or let the kids spend a little more time in Dinoland, U.S.A., then headed back to our resort for a swim or a nap. Once refreshed, we'll head back into Epcot for the rest of the afternoon or evening.
Please send the panel any additional questions you may have while planning your wonderful Walt Disney World family adventure!