Unlike senior royals such as Prince William and Catherine, Princess of Wales, or Prince Harry and Meghan Markle — who received new titles upon marriage — Beatrice and Eugenie were not granted additional titles when they married. As a result, their children did not automatically receive royal titles at birth.

Why Beatrice’s Children Have Titles — But Eugenie’s Do Not
The key difference lies in Beatrice’s marriage. Her husband, Edoardo Mapelli Mozzi, is an Italian count, which means his children inherit noble titles through his family line.
Mozzi’s father, Count Alessandro Mapelli Mozzi, previously explained: “Edoardo is the only male descendant taking the family into the next generation.”
“He is a count, his wife will be a countess automatically, and any of their children will be counts or nobile donna.”
This means that Sienna and Athena hold aristocratic titles through their father, not through the British royal system.
Eugenie’s husband, Jack Brooksbank, does not hold a noble title, which is why their children, August and Ernest, do not have titles.