Keep in mind that many car rental companies use a different sizing standard than is commonly used in the retail channels. This varies from rental company to rental company, but most rental "compacts" are a tight squeeze for luggage, although they may be fine for 4 passengers. For example, National lists Chevy Cavalier or similar as its compact (I once got a Toyota Corolla in this class) and Pontiac Grand Am or similar as its intermediate. (Full size is Buick Regal or similar, I usually look for a Pontiac Grand Prix). Even the "intermediate" Grand Am is a tight squeeze for two large and one small suitcases, which is what I have when DW and I travel with our adult DD. Given that your second group has two older children along with two adults, you probably want at least an intermediate rather than a compact for the second group, although you might get a full size with plenty of luggage space and be able to move a suitcase from one group to the other for transport to and from the airport.
Eight passengers is a tough number for car rental. Most of the minivans have a maximum capacity of 7 passengers and I share your DH's adversion to the large 15 passenger vans. Also, even a seven-passenger minivan has little space left for luggage if your group travels like mine. Thus your proposal to go with two cars is probably a good one.
Have you considered joining the Disney Club to take advantage of the car rental discounts from National? You can get a free Emerald Club membership as part of the DC benefits. Then you can rent full size for the same rate as intermediate using the Emerald Aisle and qualify for the (typically) 20% discount plus one free weekend day using DC discount codes.
Ralph