From personal experience.... DH was in school with key workers' children and nobody got ill, including staff. He was in school pre-lockdown (in the most infected borough of London) and no kids (or teachers) in his school got ill. To my knowledge, no children in my DC's school have had it, although one TA did (mildly). And this was when there was no SD, etc. I struggle to understand how it is now extremely dangerous.
On a similar note, I was still going every day to work on the tube train until the actual Monday lockdown, and although unknown to me, I had acute leukaemia and an absolutely shot immune system. And I didn't get it either.
It just slightly makes me think that.... it can't be quite as infectious as all that! 😕