Because COVID poses very little risk to children. The flu poses more risk to that age group. No one is running around saying we need to mask children for the flu and vaccinated all of them.
https://www.cdc.gov/flu/fluvaxview/coverage-1920estimates.ht...
No more than 64% for school aged individuals. So, since you're supposedly using this as a reason why we should be keeping everyone locked down then I'm going to assume you feel the same for influenza? Which means you're saying we should have shut schools down and mandated all sorts of measures to protect them since the inception of the influenza vaccinations?
Full vaccination COVID is not an achievable goal.