I would suggest shifts! Mom and Dad have proprietorship of the room for, say, 14 hours and teenagers for 10 hours. Or you might overlap the schedule a bit and say it is communal property between 3 a.m. and 7 a.m. but at 7:01 the teenagers are out until Mom and Dad have slept in, showered, dressed and departed.
The bedding is set up as twins that CAn be converted to a double but when both fold down bunks are used it makes it very hard to move around. What will probably be the best set up is a lower twin and right above it a berth that fold out of the wall or drops from the ceiling on some ships. It will be a bit crowded but doable if you really like each other. <G>
You may want to go to http://www.mytravelco.com/ and get into the "cruise" section and then the 360 tour. I think after you view what your cabin will look like, it will give you a much better idea as to what to expect.
The only major problem we've had when sharing a cabin with 4 people is bathroom time, possibly make a schedule that will work for everyone, have fun!