My first ideea is to mimic an atom and put the electrically charged ball in the middle of the carousel and make it spin, then at one of the carousel spots I would put the child with the bar magnet. Then the magnetometer in the center could measure the magnetic field of the bar magnet at the center of the nucleus B(0). Because of the spinning charged ball there will be a current giving rise to a magnetic field so there is a magnetic interaction with the magentic field given by the bar magnet.