My experience includes 5 past cruises. 3 Princess with DW, then Disney Magic, and Navigator of the Sea with DW and 2 DDs. First off, safety, we went on the Magic because we felt it was safer than WDW. The kids programs and interaction with the characters could not be matched. We were in an inside cabin on the NOS (girls were 10 & 12) and it was tight, but I feel that you dont spend much time in the cabin anyways. We have avoided Carnival because it has a party reputation. For kids, my first choice would be RCI, as it has a great program and activities for them. Also, when planning the trip, as far ahead as possible, first choose the Cruise Line and then carefully choose the ship as the different ships within the Line can offer different things. Now that our girls are a little older, we are looking at WDW in May 2007.
Northstar, as for the beds that pull down, if they are located like the ones in our inside cabin on the NOS, the only thing you have to worry about their fall is them landing on you.