In an interview with U.K.'s Hello! magazine, Mildred Baena said her son Joseph's response to the news was: "Cool!"
Schwarzenegger has split from his wife Maria Shriver amid revelations that he fathered 13-year-old Joseph with Baena, and the former housekeeper told the magazine - out Tuesday - she hopes the couple can "work things out" in time.
Baena said she wasn't initially certain Schwarzenegger was the father but started to see a resemblance in her son as Joseph grew older.
"It became more apparent as time went on," she said.
She said Shriver also grew suspicious as whispers circulated in the household over the similarities between Joseph and her boss, finally asking "point blank" if the boy was Schwarzenegger's son.
Shriver was "so strong" when she learned the truth, Baena told the magazine.
"She cried with me and told me to get off my knees," Baena said. "We held each other and I told her it wasn't Arnie's fault, that it takes two."