Since they don't want to rent a car I guess my advice is bad but, the Cheapest by far would be to rent a car for 1 day to WDW from MCO then 1 day again back to MCO from WDW.
For example, you can get a 1 day rental from Alamo for under $50 (not at the airport but Alamo will shuttle you from MCO to their rental office). You have use of the car for 24 hours. Then (1 person while the rest of the family plays) returns the car to the hotel near Downtown Disney at the Alamos' hotel rental office. Walk to DTD and take a bus to join your family. The day before you leave, go back to this Alamo hotel counter and get a car for the way back.
There are other car rental places which you can drop off on or near WDW but we've always used Alamo. Make sure to have a car reserved at both the airport and the hotel(I wish I could remember its name) so you have a car when you show up. Also, check when the hotel counter opens and closes so you don't arrive too late.....spruce