My daughter is 21 now but when she was little I bought a fitted sheet for her bed that had length wise pockets into which you slid inflatable bolsters to "encase" the child. They were soft and it worked great. I have always thought the pool "noodles" would work great for this purpose too. I like your idea as well. I would like to see a pic though. I don't have a good vision of how it will work.