It's obvious that anyone will write better in an area that interests them. That applies to a child as well. It doesn't matter which area of creative writing your child would like to involve herself in - it could be real life with a twist, it could be the next Harry Potter novel - completely fantastical - creativity grows with every kind of writing. What you can do though is to help your child along when there is a case of writer's block gumming up the works.
Do you remember the old TV show Doogie Howser M.D. about the boy genius who is a doctor? Each episode of the show always ended with Doogie writing in his journal on the computer about the lesson he learned that day. Apparently he writes in his journal to keep a promise he made his father in return for the allowance of a computer. Most parents find that it is difficult, as unexciting as it is, to have a child keep up a journal habit; and yet, it can be immensely rewarding over the long run, as a child learns to see how writing ability progresses with maturity. As long as a child knows that there is no one who is actually going to be reading their diary, their creative writing abilities can really grow well. http://www.linkinpark.com/
People read letters and other correspondence written by great people in history to see how they actually saw and felt the events that they helped shape. Writing letters can be a powerful way in which to force the creative aspect in writing. Introducing your child to this early on can be a great way to kick start the creative process.