The pilot reported that he and his 13 year old son were going for a short flight from Maryborough airport. All necessary preflight checks were carried out and sufficient fuel was on board for the intended flight. After takeoff from runway 17 at between 100 and 200 ft the engine stopped completely. Witnesses saw the aircraft enter a left turn at a low height. The aircraft then appeared to stall and hit the ground inverted. The landing gear was in the retracted position. The pilot stated that the engine failed because the magneto switches were bumped to the off position. Both switches are mounted close together in the centre of the instrument panel with other switches below them. The pilot stated that he was told in hospital that his son was waving to a person on the ground and had bumped the magneto switches off.