Their cousin, because your child is your nephew`s cousin, so your grandson or granddaughter is also your nephew`s cousin.
Your grandson and your nephew are first cousins, once removed.
You are your nephew's aunt/uncle,,, his children are your grandnieces/nephews,, his grandson is your great-grandnephew.
Your grandson and your great nephew are second cousins to each other.
Their cousin, because your child is your nephew`s cousin, so your grandson or granddaughter is also your nephew`s cousin.
Your brother-in-law is your spouse's brother. His grandson s your spouce's great nephew. (Some people say grand nephew). That makes him your great nephew, too. Your brother-in-law could also be the husband of your sister. Then your brother-in-law's grandson is your sister's grandson and your great nephew.
Your niece's son is your great-nephew. Your niece's grandson is your great-great nephew.
Your father's great grandson could be your grandson, or your great nephew.
Your nephew's great grandson is your great great great nephew and you are his great great great aunt (if you are a woman) or great great great uncle (if you are a man).
Your grandson is your sister's great nephew and she is his great aunt.
Your sister is your grandson's great aunt. He is her great nephew.
The grandson of your nephew is your great great nephew, but few people use that relationship as a form of address. Usually they call relatives by name or business title, not by relationship.
Nephew?!