As a metaphor for spiritual reality, Narnia is not seen by all. There are some who experience and see it and others who don't. When Lucy comes out of the wardrobe and insists to her siblings on her experience, she receives the same mix of reactions that Christians get from outsiders. Peter is patronizing, Susan says she's silly and off her rocker. They all think she is lying. Edmund is especially spiteful and mean about it. For a time, Narnia divides brother against sister.
As always the return to Narnia is providential, and not controlled by the children. Lucy goes off to find Tumnus and when Edmund enters a few moments later she out of shouting range. Edmund meets the witch, who is "beautiful" and "great," but also "proud, cold and stern." There are beings in the world with more power and beauty than humans, whom we are tempted to worship or follow (Scripture teaches against this tendency in Hebrews 1:4 and Colossians 1:16; 2:9-10). Like Satan, the white witch masquerades as an angel of light.
She also passes herself off as Queen of Narnia, but we'll come to that later. For now, the queen wants to know what Edmund is.