Scratch the PeaPods idea. When we walked in the nursery today after "nap" time, we found the girls on the loose and the room in disarray:
Seconds later, we discovered that Jude had actually managed to get out, too!
So it's back to mattresses on the floor for now. It took the kids an hour to fall asleep tonight (despite having skipped their nap again today), but I just went in to check on them, and everyone had found his or her own bed except for Lucy—she was sleeping by the door, poor baby!