Hi, Kara! I hope every minute of your very first Walt Disney World vacation is pixie dusted perfection!
With children this age, I would definitely do the Magic Kingdom. I think there is the most to do there for young children, and it has a great combination of classic attractions that have long stood the test of time, and brand new things that you may want to try. There are also shows, parades, and fireworks, which can really help round out a day.
As for the second theme park, that's a really tough question. If I asked each of my daughters, I would get three different answers. If I was going to choose, I might be tempted to do a second day at the Magic Kingdom. There may be some things you don't get to finish or some things your children will want to repeat. Disney's Animal Kingdom might be a good option. The kids may enjoy seeing the animals, and I think you'll find some of the best shows there. The Lion King show rivals a Broadway production, and my kids - even the oldest - love It's Tough To Be A Bug. There are some ride on attractions that might appeal to your family, including the safari, Kali River Rapids, TriceraTop Spin, or the Wildlife Express Train. The highlight here, though, is definitely animals.
Disney's Hollywood Studios has one of my favorite attractions - Toy Story Midway Mania. It's so much fun as a family to compete! You'll also find Star Tours and some great shows. EPCOT is an education in world cultures with some theme park fun thrown in.
You can visit
Disney World Attractions to see more from each park to help you decide!