Your children are cousins to your niece's children. This relationship arises because both sets of children share a common grandparent through their respective parents. Essentially, your niece is the sibling of one of their parents, making the children from both families cousins.
Nephews and nieces
Your Aunt is your children's Great Aunt.
Second cousin
Your second cousins.
Your son's children are your grandchildren.
You're not related.
your children are allowed to Mary your second cousins.your children are allowed to Mary your second 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.
aunt or uncle;) depends on the gender
Cousin once removed.
granduncle - grandnephews/grandnieces
You are their aunt or uncle and they are your niece or nephew.