No. If they are not on a room reservation, on the day they are to head to the airport, they can't ride the bus. You must be on a room reservation on the day you want to leave and go to MCO.
Because your dd's aren't going to be on a WDW resort reservation on that last day, they aren't entitled to ride the bus back to the airport. Are there no resorts, in the value catagory, that have a one night opening for them? Their only options, if they can't find a room for that night, is to take the Mears shuttle, for about $19 pp, or a car service..about $50.