FWIW - As PP's mentioned, you will need a taxi van. As a family of 5, we have never had a problem getting one when we asked.
That said, there have been a number of times (at WDW, Seaworld, and Universal) where the cab drivers have tried to put us in a regular cab with 1 in the front and 4 in the back and we had to specifically say that we wanted a cab where everyone could have a seatbelt.
The first time this happened we were on our way from POR to 1900 Park Fare and we were so surprised and ended up with DH in the front, the kids buckled in back, and me in back unbuckled. As soon as we started driving, I regretted the decision and from then on we have been very specific.