Your grand niece and your grand child share a common great grandparent. There is no generation difference. That would make them second cousins.
Your niece's son's daughter is your great grand niece. Your niece's son is your grand nephew or your great nephew, whichever term you prefer.
Your niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).
Your niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).
Your son and your niece are first cousins.
2nd cousin or 2nd cousin in law.
Your sister's son's daughter is your sister's granddaughter and your great niece. (Some would say "grand niece.") You are her great uncle or great aunt.
Your grand niece's son is your great grand nephew, or, some would say, your great great nephew. You are his great grand aunt or great grand uncle, or, some would say, great great aunt or great great uncle.
Your nephew's daughter is your great niece and you are her great Aunt (or Great Uncle).Correction:Your nephew's daughter is your grand niece. You are her grand aunt or grand uncle.
Your granddaughter and grand niece are second cousins to each other.
Your husband is great uncle to your niece's son (your great nephew).
great nephew
The relationship sequence:Your nephew/niece is a child of your sibling and your son's first cousin.Your nephew/niece's son is your great nephew (or grand nephew) and your son's first cousin once removed.The grandchild of your nephew/niece (your great nephew's son) is your great great nephew and your son's first cousin twice removed.