The child of a niece or nephew is called a grandniece or grandnephew, depending on the child's gender. That makes the aunt a grandaunt and the uncle a granduncle.
Although determining specific familial relationships can sometimes be difficult, especially when there are multiple degrees of separation between the two people, family charts can help make it a little clearer. Languages other than English often differentiate between maternal and paternal family members as well, making things even more confusing. Consulting online genealogical charts is often the easiest way to determine exactly how one person is related to another and where the family tree begins.