Louis XVI and Marie Antoinette were the monarchs beheaded during the French Revolution. They had only one son, he was about 10 at the time and he is generally believed to have died in a Jacobin prison shortly afterward. Only one daughter of the royal family survived the revolution.
Historically, the only person this tale might fit was not a "true" royal. Napoleon Bonaparte, who was later emperor of France, had a brother Jerome who was briefly married to an American woman, Elizabeth Patterson I believe her name was. They had one child, then Jerome deserted her and went back to France where Napoleon made him king of some conquered country (he was later run out of there). One of Jerome's male descendents was a cabinet secretary in the early 20th century (sorry I don't recall which post or president).