Normally you would call them by their name. There is not likely to be any relationship between you as you don't share a common ancestor.
Your niece and your son or daughter are first cousins. That would make your niece and your grandchild first cousins once removed. Your niece's child and your grandchild are second cousins.
Your son and your niece are first cousins to each other. Your son and your niece's son are first cousins, once removed.
(1) Your niece is the daughter of your sibling (2) Your niece and your children are 1st cousins (3) Your grandchildren and your niece's children are 2nd cousins (4) Your g'children and your niece's g'children are 2nd cousins once removed
The daughter of your brother is your niece.
Your niece and your child are first cousins. Your niece's child and your child are first cousins, once removed. The children of your niece's child and of your child are second cousins to each other.
Your niece and your daughter are first cousins. Your daughter and the daughter of your niece are first Cousins, once removed.
Your niece and your daughter are first cousins. Your niece's daughter and your daughter are first cousins, once removed.
Your daughter and your niece are first cousins. Your daughter than your niece's son are first cousin's once removed. Your daughter's child and your niece's child are second cousins to each other.
They are first cousins once removed. Your niece's children and your grandchildren will then be 2nd cousins.
Your great great niece is the granddaughter of your niece or nephew. Your daughter and your niece or nephew are first cousins. Your daughter and your great great niece are first cousins, twice removed.
You and your mother's niece are first cousins to each other, so your and the niece's daughter are fist cousins, once removed.
Your daughter and your niece are first cousins Your daughter and your great grand niece are first cousins, twice removed.