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.
This is anecdata though, so it doesn't really mean anything.
[1] https://translate.google.com/translate?hl=&sl=sv&tl=en&u=htt...
Robert Gallo presented [1] an interesting idea to keep the innate system boosted by taking extra doses of a vaccine (he proposes using OPV). My pet corollary to Gallo's idea is that kids may have extended protection by having been more recently vaccinated (than adults).
I wonder if the prevalence of vitamin d deficiency is lower in lower in children and hence the lowered risk and transmission.