There are a couple of things which you can do to ease the natural stress and nervousness such an important event will produce. It's too late to study for the test. But if your child has never worked a practice test, there's still time for that. Here's what I suggest you have him focus on in the days leading up to Saturday:
As a parent you must not fan the flames of nervousness and tension by asking unnecessary questions. She's a child. Let her approach this important event on her terms, not yours. Offer unqualified support and reassurance. She has to know you are solidly behind her. That will give her the confidence and clarity of thinking which she needs to be successful on the test.
Review all the SSAT test instructions and restrictions such as no cell phones and so on. Finally, relax. It's just the beginning.