Have the kids use air matresses... In a tent...
Then have them use the air mattresses in the camper, I am sure they will be OK with that...
Have you seen the portable hammocks?
Guide Gear Portable Folding Hammock - 172580, Hammocks at Sportsman's Guide They work great and do not take up that much room, not that long. And, you could use them outside. I have seen them at Samsclub and Costco, look for them in the spring.