If you are a woman, you are great great aunt to your niece's granddaughter.
If you are a man, you are great great uncle to your niece's granddaughter.
In either case, your niece's granddaughter is your great great niece.
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 niece
Your uncle's granddaughter and your child are second cousins.
You granddaughter is a great great niece of your aunt. Your aunt is your granddaughter's great great aunt.
Nephews and nieces
your 2nd niece...!
Great neice
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Your granddaughter and grand niece are second cousins to each other.
Your great nephew and your granddaughter are second cousins to each other.