We always end our trip as we begin it, at MK. We stay at the park until one hour after park closing to enjoy Main Street and get a picture with almost nobody around. We buy something totally stupid and expensive that we just have to have at the Emporium, say goodbye to the castle and leave feeling sad our trip is over but happy that we had fun and will be back someday. Unfortunately, we can't do that in May, our last night is a grad night! So I guess we'll have to mope around Epcot or MGM instead. To be honest, I have learned not to plan much in the way of ADR's the last day, we are always a bit down and it is nice to just wind down slowly before going home.
To the OP, I would maybe leave that open and let the family take a vote on their favorite park. You can always plan a nice resort dinner for early in the evening and then head to a park for a last hurrah before going home.