Because of more secondary infections.

Thanks for the question. These are different viruses, not related, although both attack the respiratory system. They have different effects on children.

First, neither are deadly for most. The sad fact about the 100-200 children who die of influenza each year, die from a preventable disease. The imperfect flu vaccine will protect. We do not know why there is a difference in the severity between the two - at least I do not know.

G. Dickinson

Since we really don't have a very long history of observation of COVID, I'm not sure we can say with certainty that your assertion is correct. So far, it does appear that the SARS CoV-2 virus doesn't cause a huge amount of disease in kids, but it is clear that kids are infected and shed the virus. So, we have a lot to learn and the only answer I can offer is "we don't know" and "we're not sure your assumption is correct." Sorry there's not more definitive answers.