I don't know how old your child is tigerhog, but anyone can sign up for the SAT (or ACT). If a child is below the age of 10, perhaps the College Board might question that child taking the test (mainly because young kids could have distracting behaviors in the test room).
The PSAT is a different matter, and it could be tough to find a school with a seat. But you can sign up a child under 13 on the paper form for the SAT. I would ask my local high school for the form - they should have them. If local schools claim they don't have them, then I would contact the College Board.
My kids have had reasons why they took some of these tests at schools other than their school, and no one questions it (nor should they). Just fill out the paper form, and make sure you put down the correct facility/school where you want to take the test.
