What is your grand child to your nephew?

Your son and your grand niece are first cousins, once removed.

Explanation:

Find the common ancestor, in this case, your parents. Your parents have two children, you and your sister or brother. Your sister or brother has a son or daughter. This child would be your nephew or niece and your son's 1st cousin. A daughter of your nephew or niece is your grand-niece, thus your son's 1st cousin once removed.

Note: Your grand-niece and your son's child would be 2nd cousins.

Hierarchy:

1. Any people

2. sons and daughters are siblings to each other

3. grandchildren are first cousins to each other

4. great-grandchildren are second cousins to each other