Hi David
We have the 390 and the inner bedroom, you cannot fit a dble mattress in there it is more like a 3/4 size.
We had to a single matress in the bedroom which meant there was space around the mattress to move around.
We thought we may have been able to use it for the children, but unfortunately only one child, goes in there.
Very easy to put up, could also be used for a storage room for clothes coming changing room.
Hope this helps.
P.S.
We used to have the 360 but upgraded to the 390 for the extra room.