Your parents are great aunt and great uncle to the children of your first cousin.
Your child and your great nephew's parents would be first cousins. Their children would then be second cousins. And if they have children, they will be third cousins.
The children of your mother's first cousins are your second cousins. The children of your mother's second cousins are your third cousins. And so forth.
your children are allowed to Mary your second cousins.your children are allowed to Mary your second cousins.
Your second cousins.
The children of your parent's first cousins are your second cousins. The children of your parent's second cousins are your third cousins.
Your mother's sister's children are your 1st cousins.
Your children and your first cousin's children are second cousins. Your children and your second cousin's children are third cousins. And so forth.
Let's assume you are talking about first cousins.If they are children of the same cousin they are siblings (brothers and sisters).If they are children of different cousins and the cousins are both children of the siblings of one of your parents, then the cousin's children are second cousins to each other.If one of the cousins is a child of a sibling of one of your parents and the other cousins is a child of a sibling of your other parent, then the cousin's children are not related to each other.If you are talking and the children of second cousins, they children are siblings, third cousins or not related.If you are talking about cousins who are once, twice or more removed from you, the possible relationships become too complex to discuss here.
Your son and your nephew are First Cousins. The children of your nephew and the children of your son are Second Cousins to each other.
The common ancestor is your parents. Your children and your sister's children are first cousins. Your grandchildren and your sister's grandchildren are second cousins. Your great-grandchildren and your sister's great-grandchildren are third cousins.
Cousin once removed.
Your uncle's grandchildren are your first cousins, once removed. They and your grand children are your second cousins, once removed