Your second cousin's niece is the daughter of your second cousin's brother or sister. Your second cousin's brother or sister is also your second cousin, so this question is the same as "How are you related to the daughter of your second cousin."
The daughter of your second cousin is your second cousin, once removed.
You and your grandmother's great niece are second cousins.
Your grandchildren and your great niece are second cousins to each other.
Your grand mother's niece is your parent's first cousin. Her children are your second cousins.
Your grand mother's niece is your parent's first cousin. Her children are your second cousins.
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 granddaughter and your niece's son (the grandson of one of your siblings) are second cousins to each other.
Your grandson and your great great niece are second cousins, once removed, to each other
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
Your niece and your daughter are first cousins. Your daughter and the daughter of your niece are first Cousins, once removed.
Yes, of course your son and your great niece are related. They are first cousins, once removed, to each other. (Your great niece's parent, your niece or nephew, and your son are first cousins to each other.)
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
They are first cousins.