Since the double beds at the resorts are bolted to the floor, I would reconsider the idea of the rollaway -- it would get very, very tight in the room.
I would suggest two options:
1) move from the POFQ to the Alligator Bayou rooms in the PORiverside resort -- you can request a trundle bed that can be pushed under the double bed during the day. (They are complimentary -- so that'd save you some cash -- but they are a little small. Perfect for someone under 12 yo.)
2) buy an inflatable mattress. We got ours at Target for $20 (we also bought a $10 battery-powered pump). It fit perfectly between the two beds and we could lift it up out of the way. Mousekeeping will bring extra pillows and sheets.