Grammar correction: Your sentence is okay, but it sounds a little better as: "Which grain is harvested by a combine harvester to make popcorn?" Or even simpler: "Which grain makes popcorn?"
Answer for a 6-year-old: Popcorn comes from a special kind of corn. Farmers grow popcorn on tall plants that look like regular corn. A big machine called a combine harvester comes through the field and picks the corn off the plants. The part we pop and eat are the little hard kernels from the corn.
- So: a combine harvester picks corn (maize).
- Only one special kind of corn — called popcorn — makes the kernels that pop.
Not all grains pop. Wheat, oats, and rice are different — they don’t turn into popcorn.