My father's father, what should my child be called?

Your father's father should be your grandfather (or grandfather with different geographical names).

Your son calls you father and grandfather (or grandfather),

Your son's father, who calls you dad, is called grandpa (some are called grandpa, some are called great-grandfather).

Your mother's father is called grandfather, and your son is called grandfather (or grandfather).

Your son calls you father-in-law and grandfather (or great-grandfather)

Your uncle's children are your cousins, and your aunt's children are your cousins.

The male children in your family are related to you, and the married women and children with foreign surnames at birth are related to you.

If in doubt, please ask.