You and your mother's brother are first cousins. There is a generation difference between your son and the daughter. That makes them first cousins once removed.
Your children and your husband's niece are first cousins. There is a difference of one generation between them. That makes them first cousins once removed.
You would be first cousins once removed. Your gran and your cousins' dad being siblings makes you and your cousins' dad first cousins. The "once removed" part comes from the generational difference between you and your cousins' dad.
Your niece and your child are first cousins. There is a difference on one generation between the two. That would make them first cousins once removed.
They would be second cousins. Your child and your grandniece's parent are first cousins. There are no 'removed' involved because the generation difference between the common ancestors (your parents) is the same.
Your son and your niece are first cousins to each other. Your son and your niece's son are first cousins, once removed.
Your uncle's first cousins are also your parent's first cousins and so are your first cousins once removed. Similarly his second cousins are your second cousins once removed.
The children of first cousins are second cousins to each other. Where two people are first cousins to each other (call them Joe and Ed) Joe's children and Ed are first cousins, once removed, and Ed's children and Joe are first cousins, once removed. Joe and Ed are in the same generation. Their children are in the next generation. "Once removed" refers to people who are in different generations and there is only one generation difference between them, as is the case with parent and child. Grand parent and grandchild, for example are two generations removed from each other.
First cousins once removed. Your child and your nephew are first cousins and there is one generation difference.
You are First Cousins
Yes, of course your son and your great niece are related. They are first cousins, once removed, to each other. (Your great niece's parent, your niece or nephew, and your son are first cousins to each other.)
First cousins once removed.