https://www.mercurynews.com/2022/09/10/harry-and-meghans-children-become-prince-archie-and-princess-lilibet/

Following the death of the Queen, Harry and Meghan’s children have become His Royal Highness Prince Archie of Sussex and Her Royal Highness Princess Lilibet of Sussex.

The change is a result of conventions created over a century ago. Under rules set out by George V in 1917, the grandchildren of the monarch automatically receive royal titles.