We almost always have a rental car...sometimes we use it alot, sometimes we don't. With a crew like that, I think DME might be easier to get to the resort when you arrive. While I have heard that
Alamo/National at the car care center will shuttle you, personlly we prefer to rent from car rentals in the DTD area. There is a Dollar at the Wyndham Lake Buena Vista and an Alamo at Buena Vista Palace Hotel. Both of these are just a 5-8 minute walk from the DTD bus stop. So, on one day, it would be easy enough for one of you to hop a bus to DTD, walk over and pick up the car. Often times we book it with a return to MCO. That way we can have the most time possible on property before we leave for the airport. With RAC, you can still check youy bags to fly home, then drive to the airport about two hours before.
This October, I couldn't find a cheap rental from the airport, so we are going the first 2 days without a car, then I have a 3 day rental for $107 from Dollar with a return to MCO.
In December, which I know is when you're going, I just got a Mid size for 8 days, pickup and drop off at MCO with a $35 coupon from Alamo Insiders for $125. If you did get a good deal for the full trip on a rental car, maybe you could take older and youngest kids and take DME and DH and middle child could go get the rental car. Fun to see who would get there first.
Just an idea.
Good Luck!