The correct response is: The children see, for the first time, how ugly Earth is compared to Uriel.
In "A Wrinkle in Time" by Madeleine L'Engle, when the children look at Earth through the Happy Medium's crystal ball, they are confronted with the harsh realities and struggles of life on Earth, which contrasts sharply with the beauty and harmony of Uriel. This comparison can be disturbing and upsetting for them.