This can be a difficult skill! You can help your child with this by asking for oral responses to the books you read at home. Just saying out loud what they liked or what they learned can be a big help in developing a plan for what to write down. If they are in the habit of orally responding to a book, it becomes easier to write a sentence in response to a book.
At the listening center, I encourage students to do their best sounding out for most words. They can use the book to copy words they need, but we've talked about not copying whole sentences. We will keep at it!