My DD was 11 when she took it. We did some prep work on both the test and going to take it. For the test, I had her read the very helpful chapter at the beginning of a carefully selected SAT study guide that explained what to expect on test day, what you are allowed to bring, what you need to bring, how the test will be run and things like that. The study guide was selected to not emphasize "how nervous you may feel" since that might have made her nervous. We used this as a jumping off place to discuss the total test taking experience step by step including that she would wear comfy clothes and a watch. "You'll arrive and we'll find the bathrooms and the test room. Someone will ask for your ID and check your name off on a list. Then you ask where you should sit. Sit there and put your clear Ziplock bag with pencils, calculator, etc on the desk and your tote bag with snack and turned off cell phone under the desk�" Literally step by step, including when she could go to the bathroom and have her snack. She also practiced the test twice with me as the pretend proctor and timer, so she could practice filling in bubbles carefully, looking at her watch and keeping track of the time for each section. We also discussed that if she finished a section early she should check her work and not look around at the other students.
Day of the test was fine. She was calm and prepared, but shocked (she told me later) that some teens came in pajamas and one teen had no pencils!
DD found it helpful to think of the test as several smaller chunks rather than one big one.
DD went/goes to a bricks and mortar school so being away from me for 3 hours was no big deal to her and we carefully did not emphasize the "stress" aspect of it.