The owner of the weight-shift controlled aircraft was being given dual instruction in circuit procedures. This involved making normal approaches followed by a low level go-around. During the fourth go-around, at a height of about 50 feet, the engine suddenly stopped. The instructor took over the controls and attempted a landing in a sparsely wooded area adjacent to the landing strip, but was unable to prevent the aircraft from colliding with a tree and striking the ground heavily. A specialist examination failed to reveal any defect with the engine or fuel system. The reason for the engine stoppage could not be positively established. However, the ROTAX 532 engine was fitted with a single ignition system controlled by an unguarded toggle switch mounted on a console in the forward cockpit. The circumstances of the engine stoppage were consistent with the ignition switch being inadvertantly bumped off by the front seat pilot.