Depending on the layout of your home, remember that any quiet and relatively dim space could potentially be a sleeping space. Is there a little used hallway that can be utilized? What about the play area, since hopefully everyone will be sleeping and it will not be used at that time. Heck, even a bathroom COULD have a playpen set up in it. LoL
If it gets too crazy with kids all in the same rooms, perhaps you can think outside the box (or bedroom, as it were)?