What significant event propels the children to action against the Whisperer?

The choice regarding the revelation of the Whisperer's intention to enslave the minds of children through his programming is pivotal as it underscores the moral stakes of the children's mission. This realization acts as a catalyst for their actions, igniting a sense of urgency and determination to prevent such a dire fate from befalling their peers and society at large. The prospect of children losing their free will and being manipulated is a profound threat, which aligns with the overarching themes of friendship, freedom, and the fight against tyranny prevalent throughout the story.

While the other options present various scenarios that could lead to action, none encapsulate the core motivation of the children as effectively. Discovering a hidden treasure or needing to rescue a peer could be exciting, but they lack the substantial ethical implications posed by the Whisperer's plan. A warning from a mysterious character could certainly provide foreboding, yet it is the gravity of the situation regarding mind control that truly galvanizes the young protagonists to unite and confront the Whisperer, embodying their courage and commitment to protecting each other and resisting manipulation.