in his/her mouth
Il mettait son doigt dans sa bouche.
He was putting his finger in his mouth.
— The Salt of the Earth (2014)
Utilise tes doigts pour les mettre dans sa bouche.
Use your fingers to put them in his mouth.
Le garçon a fourré du gâteau dans sa bouche.
The boy stuffed some cake into his mouth.
Le bébé a regardé fixement et a enfoncé son poing dans sa bouche.
The baby stared and shoved his fist into his mouth.
Il y a quelque chose dans sa bouche.
There is something in his mouth.