What’s the dynastic title name of the first born and heir/heiress of the King/Queen of the Belgians?
In the current Kingdom of Belgium, the title of "Duke/Duchess of Brabant" has been revived as a dynastic title and is traditionally assigned to the eldest son or daughter of the sovereign (even though the province of Noord-Brabant, part of the historical duchy, is now part of the Netherlands and has been so since 1648).
The eldest son or daughter becomes automatically the Duke/Duchess of Brabant when his/her parent becomes King/Queen of the Belgians; there is no ceremony or formal oath that the new duke must pass. When the Duke of Brabant loses his parent, or when he/she abdicates, his title goes to the next in line, and he/she becomes the new King/Queen of the Belgians. Unlike the office of King/Queen of the Belgians, the eldest child of the sovereign does not need to reach the age of 18 to become Duke of Brabant. However, if the heir has not reached the age of 18, he/she cannot ascend to the throne of Belgium.
When the firstborn becomes Duke/Duchess of Brabant, this title takes precedence over all other titles of Belgium. When they marry with royal permission, their spouse becomes automatically Duke or Duchess of Brabant.
As of 2019, the current Duchess of Brabant, Elisabeth (Dutch: Elisabeth Theresia Maria Helena; French: Élisabeth Thérèse Marie Hélène; born 25 October 2001), is the eldest child of King Philippe and Queen Mathilde and the heir apparent. She acquired her position after her grandfather King Albert II abdicated in favor of her father on 21 July 2013.