Edmund "was becoming a nastier person every minute," and Peter sees and rebukes him for it. Lucy calls them all beasts, because they don't believe; the beasts in Narnia are more intelligent, because of their faith.
Peter and Susan think Lucy is either going liar or lunatic. They go to Professor Digory Kirke, who gets them to consider the third option: is she telling the truth? ("Why don't they teach logic at these schools?") It would require changing some assumptions about what is possible in this world of ours... I mean, God's. "If things are real, they're there all the time." "Are they?"
And besides, the gospel writers - uh, I mean, Lucy - couldn't have made up the part about the different time in that world.
So instead of torturing Lucy for the truth or a confession, why not mind our own business? This mades things bearable for Lucy.
But providence returns them all to Narnia: "or that some magic in the house had come to life and was chasing them into Narnia."