Ahh, so it's possible a tree will fall on Rob, killing him but little Jack will be thrown clear. Not, it is hoped, into a culvert. Rob could be knocked out, fall in the culvert and drown whilst little Jack is (of course) thrown clear.
Why does no one have a proper dog in the Archers? The country side is full of labradors and spaniels. My spaniel and current spaniel x always go/went everywhere with their noses glued to the floor. A dog walker would be the obvious person to find little Jack - they always find bodies in RL and on tv murder dramas.