It will be no problem for all of you using DME for your return to the airport.
Yes, everyone is entitled ONE ride to Disney from the airport per Disney Resort reservation, and ONE ride to the airport from Disney per Disney Resort reservation. You can do it one of several ways. You can all use the rental car for your initial arrival, (and pick up your own bags), and just book DME for for the day you plan to return the rental car. (Just give them a flight number that arrives at MCO some time that morning.) Then book DME for the return also.
OR
You can book DME for your family for the initial arrival day, and book DME for yourself on the day you want to return the car (same thing, just give them a flight number that arrives at MCO some time that morning.) and you go off and get the car and your family can ride DME, and use the yellow tags and all your luggage will be transported separately. When your family checks into DME, make sure they tell the person at the desk that one member of your family is arriving on xx/xx/xx date so that they have the system correct.
OR
You can pick a rental agency that has an office near/on Disney property. Alamo/National has offices at the Car Care Center and the Dolphin, and there are a couple of others that have offices at the Downtown Disney Hotels on Hotel Plaza Boulevard. Most have shuttles that will return you to your resort.
A lot quicker than going all the way back to the airport and riding DME back in.
No matter how you do it, your family will be able to ride DME back to the airport on your last day.