We put the magnetometer at the centre of the carousel and perform 3 experiments. One wher the child has the charged ball, one where it has the bar magnet, and one where it has both. The magnetometer represents the nucleus and the child the electron.
In the first experiment, we give the child the charged ball, and let it ride the moving carousel. The magnetometer will register a magnetic field due to the moving electric charge. This simulates the orbital field.
In the second experiment, we keep the carousel stationary and give the child the bar magnet. This simulates the electron spin and the magnetometer will register the dipole contribution.
In the third experiment, we let the child roam around freely on the moving carousel, with both the charged ball and bar magnet. This simulates the Fermi fields.