A.S.
Disney is great. Try to contact Disney, as I do believe that tickets to the park are interchangeable daily.
Perhaps Magic Kingdon during the day (if you get there early the lines are so short!!!) Then back to the hotel for a nap/rest and then perhaps Epcot which is amazing and has great food, rides for the entire family and awesome fireworks!!!
If you can only go to one park, go to Magic Kingdom - it is indeed magic and is just amazing for all family members. My kids love this park. You could spend days there and still not see it all. Just amazing!!!
Have a safe and wonderful trip!!!