Game concept
Eligible Child
The game also links it as Eligible Children.
From the game's encyclopedia
Eligible Children are, by law, allowed to inherit their parent's Titles.
Bastards are not considered eligible, as well as children of certain genders under specific Gender Laws.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Eligible Child is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Eligible Children are, by law, allowed to inherit their parent's Titles." The game also links it as Eligible Children. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.