If the magnetometer gets placed in the middle of the carousel one can measure the effects that a nucleus would “feel” when different hyperfine contributions get simulated. To simulate the orbital cntribution the child could carry the charged ball around the carousel, which would induce a magnetic field. To simulate the spin dipolat contribution the child could carry the bar magnet to represent the electron with a spin going around the nucleus. I do not know how to easily simulate the contribution of the fermi contraction as the bar magnet would have to be inside the nucleus (magneto meter).