What if the managers are fired too? Should they be kept on just long enough to fire their reports? Is that somehow more humane?
Employment is a business deal. If you consider your relationship with your manager to be personal then you have an unhealthy view of your job.
As for the logistics, each member of staff can be talked to by their immediate manager, making it scale easily. Matter of fact, this once happened to me, and my boss was as clearly upset to let me go as I was to leave. But everyone being let go was informed and given their notice as soon as the workday started, so there was no uncertainty or delay.