I don't think you can reserve a rollaway per se, but since they technically require you to have a rollaway if you have 5 guests in certain room classes ...
Shouldn't be a problem.
There is no requirement all guest must be in a bed
If the room allows 5, depending on the group with littles, some parents sleep with kids in the bed
If you have children 8 and up, you can make a pallet on the floor
Renting a roll away is $25+tax per night
And that can push your bill higher than planned
I rented one once for a 6 night stay and found it was more of an aggravation as it took up more floor space
Being stuck in a room due to a nasty storm day with 4 boys was not fun
You store the roll away in your room
Just like kivara said, call star service from your room and tell them you are making pallets on the floor for x number of peeps
Request blankets to make the pallets and extra for on top of the Peeps
Sheets, pillows, towels, etc
The room phone has a button Star Service
24 hours available
Free service for all guests
Put the extra bedding on one of the beds when you leave for the parks
Staff will have all of it folded by the time you return to the room
I used the space between window and bed for one pallet and second one other bed far side
No one got stepped on during the night when nature called