Last year we rented a Mustang convertible at MCO, since my SIL and I were taking our 2 teenage girls to Orlando. My brother warned me about the trunk before I left.
My DD and I travel a fair amount and are accustomed to packing light (one carry on bag each). My SIL and her DD had enough luggage to last until the next month. The trunk barely held 2 small bags (we did fill in holes with jackets, purses, and things like that). We had to pull the front seats all the way up (not too pleasant with my long legs) and put some bags in the floor behind the front seats (the girls crossed their legs on top). We also put bags between the girls, and they held some in their laps too. (Please, no safety comments here! LOL) Of course, you have small children, so that's really a different situation.
On the plus side, we all loved the convertible and had a wonderful time. I do very much agree with DMRick, I would NEVER want to own one now (not enough room for real life!). But, it really was great for a vacation. The girls agreed they'd do the luggage deal again in a heartbeat (it was only just to the hotel and back, of course). We also took pictures - I told them we looked liked the Beverly Hillbillies (or maybe Ma and Pa Kettle!).
I hope everything works out for your trip, and you have a great time!