A child needs meaningful, not artificial writing tasks. All the time in the world won't help if there is nothing to write, nothing to say. And more practice. In helping your child to learn to write well, remember that your goal is to make writing easier and more enjoyable. Help your child spend time thinking about a writing project or exercise.
Good writers do a great deal of thinking.
Your child may dawdle, sharpen a pencil, get papers ready, or look up the spelling of a word. Do respond to the ideas your child expresses verbally or in writing. Make it clear that you are interested in the true function of writing which is to convey ideas. It's usually wise to ignore minor errors, particularly at the stage when your child is just getting ideas together. Never rewrite a child's work.