What evidence do the children find to prove the Whisperer is a threat?

The children find documents and recordings that reveal the Whisperer's manipulative plans, which serve as significant evidence of his threatening nature. This type of evidence is compelling because it provides concrete, tangible proof of his intentions and the potential dangers he poses. The documents and recordings not only illustrate the Whisperer's strategies but also give the children insights into the larger context of his actions, thereby highlighting the seriousness of the threat. By uncovering this information, the children are able to understand the scope of the Whisperer's manipulation and the impact it may have on their world, reinforcing the urgency of their mission to stop him.