Let them know why you're going to be successful and what you will add to the university. What sets you apart from the thousands of other applicants? I look at it from a business point of view. Why are you a good investment? ("you" obviously being your kid).

Depending on what the questions are this year, you can use them to mention other activities that your kid has been in. This can pad the application a little bit more and show that he/she is well rounded.

With the writing itself, you need to strive for concise and clear writing. I always told my brothers to write their essay without worrying about the word count, and then when I go back and edit it, I show them how they can scale back to the word limit by getting rid of nonessential phrases, combining sentences, rephrasing, and things like that. The main idea is to get the thoughts on paper. It helps to have a good editor around who knows your kid pretty well. The extra insight is helpful to suggest where more content can be added if the essay is way too short.

I'm 5 for 5 with university admissions so far (4 undergraduate at BYU, 1 graduate at the U of U). I'm not saying I'm right with my advice, but that's how I've always gone about writing my essays and helping my brothers write theirs.

Feel free to BM me if you have any questions.