Have you and your child write your own short stories. Trade and read. Check out these recommended reading lists . You may well be able to sympathize with your child in this area. In most cases, we hesitate to write what our brains tell us because we don't think it's good enough or it's not exactly what we want to communicate.
Writing can be like getting into a heated argument.
When you write, you have to write. You need to sort through what comes out of your pen or your keyboard. What you start out with might be the crappiest thing you've ever written, but what you eventually get to might be golden. In essence, the more you and your children write, the better you all will get at it. Let your child see you writing on a regular basis.