I was thinking about it for my 5th grader and 8th grader, but a 3 hour exam would be too much for them although they do well on practice exams (and do work on their own for hours on computer learning programs).
They both took the SCAT today, which is only two parts and an hour total. On the computer and they get a break to walk around and get a drink of water.
My main concern is not being able to be in the room with them or even nearby when they were taking a 3 hour exam. I think there is a computer version of the SAT that can be taken, that might be a better option for a younger child.