How is my Dad s cousin s grandchild related to me?

Your dad and his cousin are "first" cousins. They have the same grandparents.
You and his cousins' children are "second" cousins.
Your children and his cousins' grandchildren would be "third" cousins.

Since your dad is your father, not your grandfather, you and your dad's cousins' grandchild are
"second cousins, once removed."

How the f*ck would I know?