I would use National's Emerald Reserve to get an actual reserved vehicle (mini-van/sized or bigger) that is large enough for your immediate traveling party. Then I would spring for a roundtrip Towncar service for your folks, making arrangements ahead of time (so Mom & Dad never see the bill & don't even have to pay the tip).
Think of it...treating your parents to a towncar/limo ride from MCO to the resort. That way you would get them where they need to be with a minimum of stress (no driving to/from MCO/WDW, thereby wasting 'precious' themeparks time), plus they get the treat of being treated like stars.
Towncars run about $100/roundtrip, some include complimentary grocery stops, some have coupons available online or in guidebooks. There are several companies that DIS'ers might recommend (and some that won't have recommendations, I suppose...)
By the way, are you *sure* that a mini-van will have enough room for the 6 of you in your immediate travelling party? 6 people, 3 of whom are adult or adult-sized plus the other three who are young children with all the parapenalia that kids seem to generate... plus all the luggage...might be a bit tight in that mini-van, eh?
Also, depending on when you travel, are you eligible for the new Disney's Magical Express bus service to WDW resorts? That might be another option for some of your party.
Have fun, whatever you decide!
agnes!