I haven't read the whole thing so I apologize if this has already been suggested. Considering the ages of you kids my first reaction would be MK. However, you might not get the opportunity to see all the stuff at Epcot which leads me to the second thought. If you have park hoppers you can do what I usually do and that's to spend a half day at each one to complete your touring.
It also gives you the perfect opportunity to ride the Monorail from Epcot or MK, depending on the park you start at. Once you have been to Epcot, you will see easily if it is something that your family dynamic will work with and decide then. Just don't add the Park Hopper until you have decided that you would like to do system. It's the same price whether you do hopping everyday or just one day.