While I actually agree (probably) that shutting down schools was a mistake, correct me if I'm wrong, but the reasoning at the time wasn't because the little kids were vulnerable as much as it was that they would take the disease home to more vulnerable family members. At least, that's how I understood it.
I know my entire Covid existence during those first few months revolved around not getting infected and bringing the disease over to my disabled and comorbidity-possessing mother. I was quite sympathetic to the idea that we should avoid avenues for the disease to travel from outside the home into one where vulnerable people lived.