Et bien c 'est pour ça que l'on peut mettre plusieurs possibilités donc il faut mettre la posibilités avec "her"pour elle et la posibilités avec" his " pour lui
Possessive pronouns in French relate to the object possessed and not to the possessor therefore this phrase has three possible translations: his/her/their children.
In this case I believe "their children" would translate to "leurs enfants". Therefore Duolingo wants either his or her for the singular form
(I wrote their and it was incorrect).