Not to nit pick, but to educate -
It's ALL Walt Disney World - (the resorts, water parks, Downtown Disney, Transportation, Epcot, Disney-MGM Studios, Magic Kingdom, and Animal Kingdom). I think you meant to say that you would be concentrating on the Magic Kingdom (MK) and Animal Kingdom (AK). (By the way, there's an FAQ with acronyms that you can find here)
3.5 days in the parks, you might find yourself with a little extra time, in which case I'd recommend hitting Epcot, because that park can surprise people with how much their kids like it. I was at WDW with some nephews (and their parents) in April last year, and we did most of the day at MK, took a swim/nap, hit Epcot for one attraction and Illuminations, did Epcot the next day until the swim/nap then finished up the day at the MK, and then did AK the next day and finished with a swim/nap before they headed off to visit more family. We were able to hit most of the high points of MK and AK, and it sounds like you'll have an extra day. We also hit the headliners in Epcot.
Are you able to avoid the couple of weeks around Easter? It's really early this year (in March) and tends to really increase the crowds.