Your grand niece and your grandson are second cousins to each other
Your niece's grandson is are your great-grandnephew.
Your niece and one of your grandson's parents are first cousins. That makes your niece a first cousin once removed of your grandson.
A great niece and a grandson of the same person are second cousins to each other.
Your great grandson and your husband's great niece are second cousins, once removed.
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
Great grand niece
Your niece is the daughter of one of you siblings. Your great grand niece is the great granddaughter of one of your siblings.If your niece and your great grand niece are both descended from the same one of your siblings, then they are grandmother and granddaughter to each other. If they are descended from different siblings, they are first cousins, twice removed, to each other.
Since your niece is your daughter's first cousin, your grand niece (the daughter of your niece) is your daughter's first cousin, once removed.
Your grand-niece.
Grand Aunt or Grand Uncle depends what gender you are.
A person's grandchild and grand niece are 2nd cousins to each other.
Your daughter and your niece are first cousins Your daughter and your great grand niece are first cousins, twice removed.