You are the great-great-aunt/uncle to your niece's granddaughter. This means that your niece's granddaughter is your great-great-niece. The term "great-great" signifies that there are two generations between you and your niece's granddaughter.
Your aunt's grandchildren would be your first cousins once removed, as the your aunt's children are your first cousins.
Second Cousin
Great-niece.
Your uncle's granddaughter and your child are second cousins.
your niece
You granddaughter is a great great niece of your aunt. Your aunt is your granddaughter's great great aunt.
your 2nd niece...!
Nephews and nieces
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Great neice
Your granddaughter and grand niece are second cousins to each other.
Your great nephew and your granddaughter are second cousins to each other.