This is not a difficult assignment if you think about it in terms of creating imagery from the child's point of view: focus on what the child might see, hear, taste, touch and feel as he or she rides on the roller coaster. Most people, with some thought, can remember things that frightened them in childhood and can call on those experiences to shape a paragraph about a frightened child.
Some possible topics for focusing descriptive details might be the wind blowing in the child's face, the person's head sitting in front of the child, the aftertaste or smell of funnel cakes or cotton candy, the feel of the plastic seat on the child's legs, or the seatbelt stretched across the child's lap. Don't forget sounds and lights! Maybe the child notices the Ferris wheel lit up against the night, or hears a child crying on the ground.