The pilot reported that he had finished flying for the day and had turned off the fuel shutoff valve and tied the airplane down. He noticed a family with a 10-year-old boy at the airport and offered to give the boy a ride around the field. The pilot untied the airplane, put the boy in the back seat, started the engine, and taxied to the runway. During takeoff, about 30 feet above ground level, the engine stopped and the airplane impacted the ground and obstructions on the edge of the airport. The impact collapsed the left main landing gear and deformed the forward lower fuselage, causing substantial damage. The pilot reported that he had failed to complete a preflight cockpit check and did not turn on the fuel shutoff valve. In addition, the pilot reported that shoulder harnesses were installed and available but were not worn.