The pilot stated that he departed from his private airstrip on a local area flight. He observed thunderstorms approaching the area and the winds were increasing, so he returned to the airstrip for landing. On final approach, he noticed that some children had left a gate open and his miniature donkeys were crossing the runway. He added power to go around, but the airplane would not climb. The airplane encountered a downdraft and collided vertically with trees and the ground. Examination of the airframe, flight controls and engine assembly and accessories revealed no anomalies. Weather radar data indicated that the area overhead the accident site was associated with dissipating echoes, which were decreasing in area coverage and in intensity with time, and implied a dissipating rain shower that would be typical of prevailing downdraft activity.