The "Baldy"

Well, ever since my darling three-year-old was a baby, she has twirled her hair as a comfort mechanism. What we thought was adorable when she was little has turned into a huge bald area in the back of her head. And trust me, she didn't have much there to begin with. I noticed how bad it had gotten the other morning, and I said- "Ella! Your hair is all gone!!" She looked at me with a startled look on her face and quickly put her hand on her head to feel. She laughed and said- "No, Mommy, there's stuff up there!" Being completely unsure about what to do, but also quite certain that I better do something before she is completely bald, we went to the store today to buy a swimcap. Or should I say, 'the magic princess hat that makes your hair grow'. She was very excited about the idea at first, but I'm not sure it will make it through the night... but any advice would be greatly appreciated!!