WW2 analogy
Many children were evacuated away from the bombs. That is surely the equivalent of lockdown - an extreme measure taken to save lives.
They had varying experiences but it was traumatic for many. The host families were not always kind, were often shocked by the inner city children, and some were physically or sexually abused. Doubtless some people thought the evacuation was an overreaction to the threat, and many did return home before the war was over. Then lots of them died - around 4,000 were killed in the Blitz.
So your WW2 analogy is just not helpful.
Parents volunteering in schools
This could be done, but you need to be clear, it would be purely for childcare and socialization reasons. Babysitting is not teaching.
I know mature entrants to teaching, with challenging careers behind them, good management skills and lots of experience with children, who are incredulous at how hard teaching actually is.
To engage groups of very diverse children, manage behaviour and teach the right material pitched at the right level is complex and skilled work.
Those schools in "requires improvement" and those teachers who are managed out of the profession are rarely lazy or uncaring. They generally just haven't been able to put in place the rigorous systems and ruthless attention to detail that is needed to teach a class effectively.
We all think we have it in us to be Robin Williams in Dead Poets Society. The reality is that there is a lot of boring procedural stuff that has to be mastered before you can inspire and motivate children to learn.
If you are in a complex managerial job, think about how effective a parent volunteer would be standing in for you with no training.
So yes, consider using volunteers but don't kid yourself those children will be getting a normal education.