So how does this work?
Adding salt to ice lowers the melting point of the ice so the ice will start to melt. That is why salt is put onto icy roads in the winter.
The salt increases the energy of the particles in the ice and a change of state happens. The solid turns to a liquid because the strong forces between the particles of the solid are weakened and the particles can break free from their rigid solid structure to flow freely as a liquid.
As the ice melts, heat energy is transferred from the cream to the ice (remember heat energy is always transferred from the hotter object to the cooler object which in this case is the cream to the ice).
This transfer of heat energy lowers the temperature of the cream and it starts to freeze. Another change of state. The liquid (cream) changes state to a solid (ice cream).
So, this simple experiment demonstrates to two physical phenomena. Change of state and the transfer of heat energy.
AND you get to make fab ice cream.