I would expect the ACT math to be harder than the SAT for a 7th grader, though I expect the reverse is true for an 11th grader. ACT math is straightforward, but it covers math that most 7th graders haven't had yet. The SAT math has trickier questions, but most problems can be solved with Algebra I. I know it is supposed to cover geometry and Algebra II, but it is mainly simple concepts that kids may know by 7th grade (such as when you add the angles in a triangle, it is 180 degrees).
If you are more familiar with the SAT, take a look at an ACT-SAT concordance table. It gives you the equivalent score. My kids took the SAT in 7th and I looked up the equivalent ACT composite. Eldest had a score equivalent to 19, middle kid had 24. (Youngest is in 4th grade.).
Don't worry about the math. Eldest took both the SAT and ACT. Prepped for the SAT vocab, but nothing for the ACT. Had a 35 in math with plenty of time to check answers. Math is just a matter of learning the material. Science isn't difficult, but you need to keep moving through that section. Lots of kids have time issues on Science.
