Hi Amy!
First, I recommend ordering the free Walt Disney World
planning DVD. Then I recommend buying the book "The Complete Walt Disney World" by
Julie and Mike Neal. It is a great book for first timers as it has a lot of information without being overwhelming. It is also full
of color photos and is a beautiful book; it will help you to determine what interests you and
your family.
Each of the four theme parks are large and most 7 and 4 year old guests, and their parents, enjoy them all! I recommend spending at least one full day in each park. Epcot could easily require two if you really want to see everything in that park; of course, most guests love spending more than one day in the Magic Kingdom. Most young guests also love Typhoon Lagoon and Blizzard Beach but at those ages, they'll probably be ready to leave after only half a day at either one.
To help you plan each day, I recommend visiting the independent website
TouringPlans.com. That's the website for the "Unofficial Guide to Walt
Disney World", another good tour book. There you can look at their
crowd calendar for the days you are visiting to get a feel for the
expected crowds and to see their recommendations on which park to visit
on which day. I also recommend looking at a couple of their touring
plans which are step by step guides to minimize the time you wait in
lines. Their recommendations are based on a lot of math and years of
data collection. As an engineer, I'm a big fan of their work.
Enjoy planning and write back if you have any more questions!