Here's why it was a mistake:
* Charles Wallace's vulnerability: Charles Wallace is incredibly susceptible to the influence of IT, the evil force controlling Camazotz. He's incredibly intelligent but also naive and easily manipulated.
* IT's power: IT is a powerful and manipulative entity that can easily take control of anyone who succumbs to its influence.
* The lack of support: Without the guidance and protection of the three Mrs. W's, Meg and Charles Wallace were extremely vulnerable to IT's influence.
This mistake, however, inadvertently helped Meg save Charles Wallace. Here's how:
* Meg's determination: Because she was alone with Charles Wallace, she realized the gravity of the situation and became fiercely determined to save him.
* Love's power: Meg's immense love for her brother allowed her to resist IT's influence and fight for him. She understood that IT was trying to "take" Charles Wallace, not just control him.
* Facing her fears: Meg's courage in confronting her fears and facing IT allowed her to discover her own strength and power.
The Mrs. W's had underestimated IT's influence, leading to Charles Wallace's capture. But ultimately, Meg's determination and love, fueled by their mistake, ultimately saved Charles Wallace.