They usually use small "pack n play" style collapsible cribs which maybe are 2x3 feet. I believe it is something that you need to request before you get to the ship, as in when you book. But your agent can still arrange for it. When we cruised with our son at around 2 years of age we got a triple cabin and kept the beds split. Our son got one bed to himself and I used the pulldown bunk.