Is there a kind of steel that stands up well to your parents slicing everything on China plates and levering pumpkins in half etc
No, there is no steel that can withstand such abuse. If education is not a realistic option, set them up with cheap serrated knives and hide your good sharps when they come to visit.
Edited to add that the cutting on china plates is unfixable. Big thick knives can withstand torquing pumpkins apart.