I think they should be able to write as well as they are expected to talk.
When my 3rd grader speaks to me, she forgets to express the subject. I stop her mid-sentence and make her provide me the subject. Is there any wonder how she writes a sentence.
At minimum, her sentence/paragraph should expres the 3 W's.
I will pull the "What your 3rd grader should know" book in a bit and add to my response. It will be late tonight, so if you are on the East Coast, expect to see it tomorrow.
From the book:
It starts out with writing a report. After reading a short story about box turtles, they suggest you plan your report. Find the ideas you want to write about. They suggest, 1. What box tutles look like. 2. Where box turtles live. 3. What box turtles eat. It is suggested to start a paragraph with a sentence that states the topic or main idea of the paragraph, followed by a few more sentences to explain the idea and give examples.
Each time you start writing about a new idea, you should begin a new paragraph, shown by indention.
They reiterate what I said earlier. "You know sentences, you speak and write them all the time. A sentence is a group of words that expresss a complete thought. Every sentence has a subject and a predicate."
Have her write a sentence, then show her how to spruce up her sentence with some nice adverbs.