Yppej, math - 31% of children TESTED. They did not test 100% of children in Florida. So the known positive percentage is 31% multiplied by the percentage of children in Florida tested (which is number tested divided by total number of children times 100.)
I’m going to guess that given the unpleasantness of the test and the uncooperative nature of children, most people don’t get their kids tested unless they are pretty sure they are sick.