What is Mr. Curtain's primary method of control over the children?

Mr. Curtain’s primary method of control over the children revolves around the Whisperer, a device that enables him to manipulate their thoughts and influence their actions directly. This method illustrates his reliance on covert psychological manipulation rather than overt threats or physical force. By altering the children's thoughts, Mr. Curtain can achieve compliance and sway their loyalties, making them more susceptible to his overarching plans. The use of the Whisperer thus represents a sophisticated and insidious form of power, allowing him to maintain control without needing to resort to more primitive or violent methods like punishment or physical restraint, which might breed rebellion or resistance. This form of control also emphasizes themes in the story about the significance of free will and the dangers of manipulation.