Hi Karen:
This sounds like a fun vacation with your family.
Here's my gut: don't decide in advance. I would head to Magic Kingdom on your first day. There is so much to take in, you won't want to rush through it, especially with young children in your party. At the end of the day, if you feel that you didn't fully see all that you wanted to see in Magic Kingdom, then return to the Magic Kingdom for the second day to hit the attractions you missed (and re-ride the ones you loved). Magic Kingdom is so large, it's generally considered a "two-day Park." However, if after the first day, you feel you've experienced enough of Magic Kingdom, then choose to visit a different Disney World Park your second day.
For a second Park, I would choose Epcot for its number of attractions and variety of activities. At Epcot, my family always takes in Spaceship Earth, Living with the Land, Soarin' (your younger children may not hit the height requirement so I would take advantage of Rider Swap), Turtle Talk with Crush, the Seas with Nemo & Friends, the attractions inside the Innoventions pavilions (by boys are obsessed with The Great Piggy Bank Adventure), and Agent P's World Showcase adventure. We also tour the countries of World Showcase. Every family is different and if your family LOVES animals, then Animal Kingdom may be a better second park for you. Also, many families enjoy Hollywood Studios, especially because of Toy Story Mania. But for me, my gut tells me two days at Magic Kingdom or one at Magic Kingdom and one at Epcot.
Enjoy your vacation.