Well, as a follow up, my seventh and ninth graders both sat for the SAT today. They didn't want me to walk in with them, and all went well.
My DS15 says he had at least one question he wasn't confident of the answer. DD13 said it was a few more that she was uncertain of, and she particularly didn't like the dense nonfiction passages about science and politics.
Chances are probably good that they didn't get DYS qualifying scores, but at least it was a chance for them to experience some independence, and to stretch themselves a bit, and they feel good about it. DS is looking forward to studying for a "for-real" SAT around his junior year. DD says it would have been a good experience IF she hadn't been annoyed that she was missing pottery class. (We had signed her up for the weekly Saturday morning class after the SAT was scheduled, and it didn't occur to me to just reschedule the SAT, until it was too late to do so.)
Thanks again for the advice and encouragement!