You are Great Aunt to your niece's child. The child is your great niece or great nephew.
If your niece had children, you would become their great aunt.
Your children and your niece are first cousins.
Your children and your niece's children are first cousins, once removed.
The children of your niece are your great nieces (the girls) and your great nephews (the boys)
Niece and nephew. They are the children of your (or your spouse's) sibling.
great great nieces
Your daughters are great aunties if their nieces have kids. Your daughters kids are your grand children.
Your brother-in-law's children are your nephews and nieces.
Your brother's children would be your nieces and nephews.
He is their Uncle and they are nieces and nephews.
Second cousins.
Your niece's children are your great nieces and great nephews.
Your sister's husband is your children's uncle. They are his nieces and nephews.
Your great nieces or newfews
Nieces and nephews are the children of your siblings.
Nephews and/or Nieces
Your great nephew's children are your great great nieces and great great nephews. You are their great great aunt or great great uncle.
They are your nieces and nephews and not your cousins