Who were Medusa's grandchildren?

Yes.

Where Medusa was never a mortal woman, her parents are Phorkys, god of the hidden dangers of the sea and Ceto the goddess of the dangers of the sea personified as sea-monsters as well as sharks and whales; Pontus and Gaia are the parents of Phorkys and Ceto, and so grandparents of Medusa.

Another parent of Medusa is credited as Gorgon, whom Zeus killed at the start of the Titan war and crafted the Aigis (or Aegis) cape from his/her skin. This creature's gender is undetermined; and may have been the same as Aix, the daughter of HeliOS; Gorgon is most often called the offspring of Echidna, mother of monsters, and Typhoeus her mate.

Giving the grandparents of Medusa as:

Pontus and Gaia

Echidna and Typhoeus

HeliOS and Krete