Your granddaughter and your grandniece are second cousins or third cousins to each other
Your uncle's granddaughter and your child are second cousins.
Your granddaughter and grand niece are second cousins to each other.
Your great nephew and your granddaughter are second cousins to each other.
Your children and your husband's niece are first cousins. There is a difference of one generation between them. That makes them first cousins once removed.
You and the granddaughter of your great grandmother's first cousin are third cousins, once removed.
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.
You and your father's cousin's granddaughter are 2nd cousins once removed.
Your brothers granddaughter is your grandniece.
They would be your parent's first cousins once removed. So they would be your first cousins twice removed. They are two generations away from you so they are twice removed.
It could be one of two answers. They could be third cousins. Or they could a first cousins four times removed.
Your grandson and your sister's grandson are second cousins to each other.