Thanks for all the replies. I'm sorry I haven't been back here in awhile. I found some Dora the Explorer sheets that worked great. This turned out to be no big deal at all. She didn't need a rail. The bed is pretty low to the ground. She has fallen out a couple of times, but wasn't hurt, just confused. She has even learned to use the pillow. She gets up sometimes and turns her light on, but I just warn her to go back to bed and she stays put after that. This has been a cake walk compared to oh, say, potty training.
One thing I found that was helpful: I'm a night owl and usually I'm awake and I hear her if she falls out or gets out and I go in and put her back in bed. But I leave a warm blanket on her floor so that if she does get out while I'm asleep, she has something to curl up in.
A couple of times I have come in and found her all curled up in the blanket on the floor. Once she was even under her bed. It gives me peace of mind knowing that she will be warm even if she's too sleepy to crawl back in bed. I also leave cushions and a dog bed (a new one, unused by a dog) on the floor.