When they arrive at CENTRAL Central, the children interact with the red-eyed man, an older person seated on a chair on a platform. His voice, which enters their minds through telepathy, seems kind and soothing to them. Yet his intent is to brainwash them, and he uses deceit to try to get their brains synchronized with his own. He attempts to have them recite the multiplication tables with him, and the rhythmic singsong of this activity starts to lull them under his control. The children have to fight by crowding their minds with their own words and emotions to keep him away.
This, along with other indications, such as the way the Camazotz children they passed earlier do synchronized ball bouncing, suggest that Meg, Calvin, and Charles Wallace are going to face the challenge of fighting off mind control. The evil force that has overtaken the planet wants to rob them of their individuality and put them under its power so that they can't think or act independently.
We can predict that fighting this challenge will not be a matter of physical strength or physical weapons. They will have to fight with their minds and emotions—the combination of which comprises their souls—to complete their mission of rescuing Meg and Charles Wallace's father.