If we let the electrically charged ball rotate with the carrousel (so let’s say te child sits on the carrousel and holds it) we can recreate the orbit of an electron in an atom. The charged ball creates a current which induces a magnetic field which can be measured by the magnetometer. This represents the orbital contribution to the magnetic hyperfine field.
The dipole contribution can be recreated by replacing the ball by the bar magnet.
The Fermi contribution we want to recreate the situation where the electrons are inside the nucleus. The child that is holding the ‘electron’ will need to be closer to or even at the center of the carousel.