I can believe that there exists a nuclear core in the planet Earth, also I would believe that a similar core could manifest itself inside other planets. But what I do not believe is that the molten core of a planet has any significant effect on the magnetic field that surrounds the said planet. In my opinion it is the orbiting charged particles (VanAllen belt) that acts like a current in a coil of wire that creates the magnetic field around Earth. This ‘cloud’ of charged particles moving at a high speed around the planet is created by and very susceptible to the whims of the ‘solar wind’. In other words if the ‘wind’ stops blowing and the particle density or velocity decreases then the global magnetic field will be affected by these changes. If the direction of the ‘current’ changes then so will the polarity of the magnetic field.
Other planets may also have such a charged field around them.
I do not know of any law of physics that would prevent the afore mentioned phenomenon from happening.
Cheers :^)