You can find My Melody's Grandfather ingame in HKO in Paris near the Palaise de Versailles. His wife, My Melody's Grandma, is standing close to him, as they are still a loving couple at their ripe old age.
Melody's Grandpa knows a lot about pets and how to breed them. This nice old fellow will even give a pet to you for free!
Grandfather might be old, but is still very adventurous and active.