Your great nephew's child is your great great niece (if a girl) or great great nephew (if a boy).
Your nephew's children are your great nephews and great nieces (or grand nephews and grand nieces).
Your nephew's child is your great niece (if a girl) or your great nephew (if a boy).
Your brother's child's children are your grandnieces or grandnephews.
you would be the child's great aunty or uncle Therefore, they would be your great niece or nephew.
Second cousins.
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
Your great uncle's child would be your second cousin. I think technically it is your first cousin, once removed! Second cousins, are cousins of cousins.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
If your great uncle is the brother of one of your grandparents, his wife is your great aunt but their nephews and nieces onher side are not related to you. Those nieces and nephews (and their children the great nieces and great nephews) have no ancestor who is also one of yourancestors.
Your great aunt's husband is your great uncle, but his siblings and their children and grand children are not related to you. The grandsons of your great aunt's husband's siblings are his great nephews (not related to you). On the other hand, all of your great aunt's great nephews are also her husband's great nephews. These great nephews are the grandchildren of your great aunt's siblings. They are you (if you are a boy), your brothers (if you have any), your male first cousins and your male second cousins on whatever side of your family your great aunt is on.
The children of your grand nephew (or great nephew) are your great great nephews and great great nieces (depending on whether they are girls or boys).
Great nephew