"their (plural)" in French

“Leurs” agrees in number with the noun that follows, not with the number of people we’re talking to. It is the same in both masculine and feminine forms.

e.g:

Marie asks Michel : “What are the names of Sophie and Gilles’ children?” (Comment s’appellent les enfants de Sophie et Gilles ?)

Michel replies: “Their children are called Julien and Claire.” (Leurs enfant s’appellent Julien et Claire.) because children (enfants) is a plural word.