Erica,
While all the parks are good, if I were in your shoes I would do this;
Day 1: Magic Kingdom
Day 2: Hollywood Studios
Day 3: Open (see below)
Day 4: Magic Kingdom
The Magic Kingdom is your most important park, by a long shot. Epcot, while perhaps my favorite, may not have as much for your kids as the others. Animal Kingdom, which I also like, just has fewer attractions in general. Hollywood Studios, with the inclusion of both Disney Junior and Pixar characters, is a good bet for your kids. But the most important thing is that you want to start and end your visit at the Magic Kingdom.
Why do I leave Day 3 open? I would say that after your first two days, you will have an idea if you want to add yet another day to the Magic Kingdom, of if you want to explore another park. If after 2 days you feel like your last day being at the Magic Kingdom will be enough, then go to;
- Animal Kingdom if your kids are big fans of their local zoo
- Epcot if your kids love Nemo, and are willing to try new foods with you in the World Showcase
I hope that this helps? Write me back with any follow-up questions, including travel dates, and we can refine the suggestions!
Have a magical vacation!