Brian,
I also have three children under 9 so I know this routine
very well! We generally allow at least two days each to visit Magic
Kingdom and Epcot, and one day each for Animal Kingdom and Hollywood
Studios. On any additional days we might have we visit resorts, Downtown
Disney, or head back to a park we want to enjoy just a little bit more.
You are wise to be planning your itinerary now, as spring break can be a busy time to visit Disney. One thing that I strongly recommend that you do is to make your dining reservations ASAP. Disney begins accepting them 180 days in advance so you are already able to book, and super planners like myself will have already done so. The sooner you call to book, the better chance you have of getting to dine when and where you want.
In order to book dining, though, you will need to know where you will be each day. For this, I like to visit the
Calendars page and look at park hours for my dates of travel. Knowing that helps me to plan which park we will visit on each day, and from there, where we will need to dine. (I typically try to eat in the park or at a nearby resort so we don't waste precious park time driving back and forth all over property)
For Princess specific dining, we like breakfast in Epcot at Akershus (Norway), lunch in the Magic Kingdom at the castle, or dinner in the Grand Floridian at 1900 Park Fare. I would also consider a meal at Chef Mickey's in the Contemporary or Tusker House in Animal Kingdom if your children would also like to meet Mickey and friends.
Hope that helps you get started, and of course feel free to send in any other questions that pop up as you plan your meals or park days. We would love to help!