Hi Mitchell!
A week at Disney World! Terrific! Magical times await...
2 days at Magic Kingdom should be more than enough. If you reserve 3 days, you may get that "Weren't we just here?" feeling by your 3rd visit (smile).
The only other park I could see splitting would be Epcot. If you plan to hit all the major rides at Epcot, then you could feasibly split one visit as the "rides" visit, and the other as the "worlds" visit, by hitting just the World Showcase one day and immersing yourself in each country.
At Epcot alone, you have attractions "Honey, I Shrunk the Audience," "Mission: Space," "Soarin," "Spaceship Earth," "Test Track," "The Seas with Nemo and Friends," all of which your 7-year old should enjoy!
That would give you 2 days at Magic Kingdom, 2 days at Epcot, 1 day at Animal Kingdom, and 1 day at Hollywood Studios. 6 days total, with 1 day R&R. I always recommend for any trip 5 days or longer, you should take one day to just enjoy the resort. The resort is normally the largest investment one makes on a Disney trip, you have to take just one day to enjoy that investment! Each resort also has a number of amenities and activities, so don't worry about getting too board hanging back.
Best Wishes on your March visit!