Why isn't Charles afraid of Mrs. Who?

Charles Wallace isn't frightened by Mrs. Who because his dog doesn't bark or growl at her.

Charles took her by the hand and dragged her away from the house. Fortinbras ran on ahead, and Calvin was close behind them. “No,” he said, “I don’t either, yet. Not quite. I’ll tell you what I know as soon as I can. But you saw Fort, didn’t you? Not a growl. Not a quiver. Just as though there weren’t anything strange about it. So you know it’s okay.