I have several friends who work these, every time I've heard about them it's been groups of schoolage kids (usually a school field trip or scout troop), with grown-up chaperones. I don't know that an individual family can't do it, just saying I don't think that's the norm. There's a tour and pizza dinner first, then everyone spends the night in the exhibit. My understanding is that the program is geared toward kids--no offense, but I don't think a lot of adults would volunteer to spend a night in a sleeping bag on a cement floor!
In addition to Penguin Encounter, I know they are also done quite often in Shark Encounter and Wild Arctic. You sleep on the floor, within sight of the aquariums/exhibits. No beds, and if you need to go to the bathroom in the middle of the night, you have to wake up one of the SW guides and be escorted half-way across the park. It's not quite the same as a night in Cinderella Castle, lol.