Hi Marcie,
Thanks for visiting the Disney Parks Moms Panel with your question! There are a couple of great options available for your luggage on your check-out day.
Bell Services at your Walt Disney World Resort will be happy to store your luggage for you on your check-out day. Simply take your luggage to Bell Services in the lobby of your resort, or call them to come assist you with your luggage whenever you are ready to check out. They will store it for you. When you return to catch your Magical Express Transportation back to the airport, simply head back to Bell Services to retrieve your luggage and take it with you on the Magical Express.
Another option - and in my opinion, and even easier one if you are able to take advantage of it - is to use
Resort Airline Check-In Service. If you are flying with a participating airline and meet the requirements, you can check your luggage right at your Walt Disney World Resort! You must do this at least three hours before your flight departs, and you will also receive your boarding passes. Your luggage will be taken to the airport for you and you will not need to worry about it again until you claim it at your home airport. See the link that I provided for details, including to check if your airline participates in this service.
I hope that this information helps, Marcie! Please do not hesitate to ask if you have any further questions.