If your child is 7th, 8th graders or lower, ACT and SAT are above level (at least 2 grades) testing. All Gifted Talent search (John Hopkins CTY, Duke TIP, NUMATS, even DYS etc... use as measurement of giftedness.
It will not give IQ per se but you can measure your child against national average of juniors, seniors and peers. There is SAT to IQ conversion chart and ACT to SAT conversion chart on the web. By no means, they are accurate especially not for 8th grader. IQ is mental age/chronological age and ACT/SAT is meaant for age 16-18.
http://www.ivywest.com/acttosat.htmhttp://www.sq.4mg.com/IQ-SATchart.htmMy suggestion is to enter talent search and you get a better idea of the level of giftedness. Better yet, take full IQ test like WISC, WJ or SB. But they could cost you a bundle :-)