You would put the magnetometer in the centre of the carousel, as the we want to know the magnetic field at the centre of the atom i.e. at the nucleus. If you have a bar magnet that a child is holding on the carousel, then wether or not it is moving, the magnetometer will measure a magnetic field, the dipolar contribution. If the child is holding a charged ball and the carousel is moving, the moving charge will create a magnetic field, the orbital contribution. The fermi contact contribution is ifficult to describe as it requires spin. But if you would have like 1 object that is both a bar magnet and electrically charged, like an electron, on the carousel. When this object is at the center of the carousel, there would be an additional field, but you can not explain it clasically.