There's an intuitive model that makes some sense of this finding: Masks, air filters, and distancing are effective at preventing transmission by reducing -- not eliminating -- viral load. What suffices to prevent infection from a 15-second encounter with a stranger in line at the supermarket, may not be enough when kids are in the same room breathing the same air 6 hours of every day.

We now know what it takes to protect a person from COVID in an environment where they are likely to be exposed to significant virus concentrations; unfortunately, getting kids to scrub up like COVID ward nurses is probably not realistic.