What is the expected score for a 7th grader taking SAT?

There's no single "expected" score for a 7th grader taking the SAT. The SAT is designed for high school students, typically juniors and seniors. A 7th grader's score will almost certainly be significantly lower than the average score for high school students, simply due to the lack of exposure to the material covered in high school coursework.

Trying to predict a score would be meaningless. It's much more helpful to focus on what the 7th grader is learning in school and how they can improve their skills in math and reading comprehension. Taking a practice SAT could give them a sense of the test format and identify areas for improvement, but the score itself shouldn't be interpreted as a measure of their overall academic potential at this age.