only child
Notre fille unique est morte d'un cancer.
Our only daughter died of cancer.
She is an only child.
Est-ce que toi aussi tu es fille unique ?
Are you also an only child?
Is she your only daughter?
Dis à papa qu'être fille unique te va bien.
Tell Dad that you're perfectly happy being an only child.
— Il était Temps (2013)
Tu es gâtée parce que tu es fille unique.
You're spoiled because you're an only child.
— Entretien avec un vampire (1994)
She is their only daughter.