Asbestos is unnecessary; the fire is made with the embers of burnt wood that at the time of the fire walk have been reduced light porous carbon that is only burning in a thin layer at the surface. This thin layer is indeed very hot, but as soon as a foot cuts of its oxygen supply it stops burning and the extremely low specific heat capacity of the material means that very little heat is lost to the foot. In short no magic just Physics. Now if they were to walk on rocks at the same temperature; that would be a very different story. Still good luck Mike and I hope you make lots for the charity.