My DS (2 1/2) slept on the couch....his first time not in a crib. The couch is not high off the floor, and we just tucked the sheets in tight and put a pillow on the floor just in case (it might be about a 12-18" drop) Well, one night on the Magic, I heard a slight thud. He had rolled out, but never woke. I just put him back in and tucked him in tighter.
I think they have rails if you ask for them, but we gave it a shot w/out and had no problem.