we can place the charged ball in the center of the carousel where this ball can spin on its own axis. This acts as the nucleus with spin I.
the child can carry a bar magnet (i.e electron) and orbit around the our nucleus. Since, an electron can interact with with its magnetic field during this rotation, this gives rise to a total angular momentum J. a magnetometer can be used to see how J orients itself with respect to I to give rise to hfs.