How would an American child intrepid this?

How would an American child intrepid when her father hands her a Greek pport proving she is a Greek national. He olso hold a US pport proving she is a US national. She always taught that is was a US national. Her father says that she is Dual US/Greek national because her father’s mother emigrated from Greece to the USA when she was a child? She does not know that her mother is aware of this. She thinks her mother does not know this, but her mother does know this.