Complex sentence: Once they got to the field where the Fourth of July event was being held, a special challenge was presented-they had to give a three-minute demo to all 150 people and the Selection Committee, before handing out the food.
Split into simpler sentences: Once they got to the field where the Fourth of July event was, a special challenge was presented. The challenge was that they had to give a three-minute demo to all 150 people and the Selection Committee, before handing out the food.