Hi Kevin,
I would recommend going to the Magic Kingdom on Saturday. Before you go down choose a five or six attraction at the Magic Kingdom that are your must dos. If your six years knows about the trip in advance let him or her help make the list. You should have time to do more than six attractions, but this way you won't feel rushed or disappointed that you didn't get to see everything.
On Sunday, go to either Epcot or Hollywood Studios. You have the park hopper so you could do a little bit of each park. My family will often spend a day at Hollywood Studios, then have dinner at Epcot, and end the night watching the fireworks from
IllumiNations: Reflections of Earth which are also at Epcot.
On Monday visit Animal Kingdom during the day and then travel to the Magic Kingdom for dinner and the
Wishes Nighttime Spectacular fireworks.
I usually find that the Magic Kingdom crowds are a little thinner on Tuesday and Wednesdays. By mid-week your kids will have a good idea which park they like the best. You may want to let them pick out where you go during those days. You can make dinner reservations at a WDW resort so that you are not committed to a specific theme park.
I hope that helps. Be sure to review the WDW website to get a feel of what attractions you will like the most. Just try not to squeeze too much into your days. The best WDW vacations are the ones that enjoy at a relaxed pace.
Thanks for the question - Michelle