Very simple really. All the school wants to do is determine how well your child can express herself in her writing. Many schools will split this part of the application up into a series of questions. On The Madeira School application, for example, she is asked to answer five questions in Part 2. By the way, Part 2 of Madeira's application is to be completed by the candidate. This is very important. The school wants to hear what your child has to say. Not what her uncle or father has to say.
One thing you must never do, no matter how tempted, is to use the services of an essay writing company such as EssayEdge. Most of the time it isn't possible anyway, because the two places where an essay is required are on the SSAT test itself and during the interview at the school. So, put that thought out of your mind right now. The school wants to hear what your child thinks, it wants to see how she writes and all in her own words, not somebody else's.
The secret to writing effortlessly is to practise as much as you can. Encourage your child to keep a journal. Show her how to write a blog. Get her to write. Every day ideally. Show her how to model her writing after the best examples. Yes, that means that she needs to read great literature. As in books. Don't just watch the movie version. Get in the habit of setting aside time for reading. Make it an enjoyable experience with no distractions such as your iPod or TV on in the background. If you haven't bought a Kindle for her, do so. She'll take it everywhere with her and begin to read voraciously.
She's going to be nervous. Tell her not to fuss about grammar and syntax. If she has those skills, then she needs to use them. It's more important for her to express herself. The school wants to see and feel the real her, not some persona which she think they want to see. She is unique. Let that uniqueness shine.