* "Accueillir" (to welcome) is used when referring to taking someone in and providing them with care. For example: "Ils ont accueilli un enfant dans leur famille" (They fostered a child in their family).
* "Placer" (to place) is used when referring to the act of putting a child in foster care. For example: "L'enfant a été placé en famille d'accueil" (The child was placed in foster care).
* "Famille d'accueil" (foster family) is used to refer to the family that takes in a foster child.
* "Enfant placé" (foster child) is used to refer to a child in foster care.
The best translation will depend on the specific situation you are describing.