Last week we were riding the Aladdin ride at MK and we wanted to all squeeze into the same seat...my husband, myself, and 17mo old son. We sat him on our laps, which then caused the fabric belt to be about an inch short. As we were fiddling with it, a cast member walked by and said they had seatbelt extenders and asked if we wanted one. It was just like the extenders on airplanes, and let us breath while still being secured.
I'm not sure about all rides with those belts, but it's nice to know they might have them for the belted rides.