What is the ruling on an aunt (maternal) saying to her sister's son: "May Allah separate us and your money," and what is the expiation for that if she wishes to retract it, and does the ruling apply to her children?

The aforementioned statement carries no consequence, so the woman can retract her words and take from her niece's son's money if the taking is lawful. She owes no expiation because this phrasing is not an oath. There is no harm in her taking from her children's money or her children taking from the cousin's son's money if that is done lawfully.