You can help the child come up with a topic by thinking about his own experiences or books he has read. The child is likely to write several drafts of the same piece. Writing evolves over time so first drafts differ greatly from final ones. During the drafting step, young children may talk and draw as much as they write. Many times their first drafts are quite short.
First drafts are likely to have grammatical, spelling, and punctuation mistakes.
At this point in the writing process, you do not need to correct these mistakes. The child will correct these mistakes as she revises, rewrites, and edits subsequent drafts. You can support the child by responding to the content of her drafts and asking questions to help her focus on how to express her ideas clearly. Keep your expectations realistic and tailored to the child's ability. Remember the importance of keeping the task at a level where the child can experience success and sustain motivation.