You're referring to the 1989 French film "La Vie est un long fleuve tranquille" (English title: "Life Is a Long Quiet River") directed by Étienne Chatiliez.
(1988) FRENCH DVDRip
Life Is a Long Quiet River Year: 1988 Director: Étienne Chatiliez Genre: Comedy, Drama, Satire Language: French (VF) Format: DVDRip Video: MPEG-4 / AVC Audio: French MP3 / AC3 2.0 La Vie Est Un Long Fleuve Tranquille -FRENCH--DVDRIP-
A key target of the film is the French republican ideal of égalité des chances (equality of opportunity). When the social worker reveals the truth, she assumes that a simple correction will restore justice. Instead, it creates only misery. The Le Quesnoy parents are horrified not by the loss of their biological son, but by the contamination of their lineage. The Gros-Dubois parents see Momo’s newfound wealth as a lottery win, leading to vulgar consumption (buying a racehorse named "Bourgeois"). You're referring to the 1989 French film "La