You can’t prove that what we did to children saved millions and millions of lives.
Frame your argument in terms of quality adjusted life years and the whole thing falls apart. The median age of somebody dying of Covid was higher than the average human lifespan. What kind of sick demented society steals two or more years of little kids childhood for that?
Oh can you prove that keeping schools open would NOT have resulted in even more mass death than we have already experienced? What about the millions of children who lost parents?