Florida's covid data shows that they're testing far fewer kids then the rest of the population [1]. The 30% figure apparently comes from [2] which shows that of the kids that are getting tested, a significantly larger proportion are covid positive than the normal population.

To be blunt, I don't think this is anything new or surprising. Kids are staying home more this summer than ever before, and with limited social interaction they're less exposed to other respiratory illnesses that are circulating in the general population. Hence, those that are coming down ill right now are more likely to have covid than not.

[1] http://ww11.doh.state.fl.us/comm/_partners/covid19_report_ar... [2] http://ww11.doh.state.fl.us/comm/_partners/covid19_report_ar...