Final answer:
To find pairs of kings or queens where one was the great-grandchild of the other, one would need to study royal family trees. An example would be Queen Victoria as the great-grandmother of King George VI. Detailed historical records are required for an exhaustive list.
Step-by-step explanation:
Finding all the pairs of kings or queens where a was the great grandchild of b would involve an examination of royal family trees throughout history. This history question can explore various dynasties and the lineage of monarchs. For example, Queen Victoria of England, who had many grandchildren that took thrones across Europe, can be a starting point. Her great-grandson would be King George VI. Another potential search could involve looking at the Bourbon or Hapsburg dynasties which also had extensive royal connections across Europe.
However, to provide a precise list, one would need access to detailed historical records regarding royal family lines, which is beyond the scope of this platform. Royal genealogy is a complex subject because of intermarriages and claims to thrones that could sometimes involve contested successions. The listing of such pairs would be extensive and would likely not be exhaustive due to the variable nature of historical records.