The pull down bunk comes from the ceiling and is up against the wall on one side. The other side has a built in guard rail.
The "twin bed" is actually a couch that is made into a bed and is also against the wall already.
You should also note that since you are on a ship, the furniture is secured and is not moveable. There is also no room in the cabin to rearrange furniture. You will not be able to rearrange the furniture, with the exception of the small table, but even that is exceptionally heavy so that it will stay put as well.
Your child will be fine with the way Disney has it all set up. YOu will not have to bring a guardrail.
Ann-e-mator