I think this is far fetched. Many viruses are much more harmful to adults than to kids, e.g. measles. Further, among the adults who have a bad case of COVID-19

I think this is far fetched. Many viruses are much more harmful to adults than to kids, e.g. measles. Further, among the adults who have a bad case of COVID-19, the vast majority had some relatively serious underlying condition (which is often correlated with age, but it seems to be the underlying condition, rather than the age, that is a better predictor of bad outcomes). Additionally: there are many countries in which D-fortified milk (and even milk consumption among kids) is not nearly as common as in the US, but the age distribution of bad COVID outcomes seems to not change.