Charles Walluas says that they can’t make decisions based on fear. Do you agree? Please explain.

I agree because IT is doing its best to get control of the children's minds. IT is already in control of their father. Though fear tempts the threesome to turn and run away from Central Central Intelligence, as Charles Wallace notes, they won't make the decisions that will free their father if they give into fear. They need, instead, to face their fears. As Calvin says, quoting Mrs. Who, who was quoting Franklin Delano Roosevelt, the only thing the children had to fear was fear itself.
IT is trying to hypnotize the children. If they keep thinking, rather than allowing themselves to be paralyzed by fear, they can block out IT and avoid mind control. They have to keeping thinking thoughts that are not IT's thoughts and that means overcoming their terror.