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you would be the child's great aunty or uncle
Therefore, they would be your great niece or nephew.
You would be his great aunt. To you he would be your great nephew.
Your nephew's son is your grand nephew or great nephew. Some people say one, some say the other.
Your nephew's son is your great nephew. Your niece's son is also your great nephew.
Your nephew's child is your great niece (if a girl) or your great nephew (if a boy).