Borrowed from French enfant de chœur.
enfant de coeur
- (obsolete) A child who sings in a choir, typically quiet and well-behaved.
- 1919, Ronald Firbank, Valmouth, Duckworth, hardback edition, page #88:
- "Nowadays, the young people sit while their elders stand!" the Abbot succinctly said, riveting a curly-pated enfant de coeur lolling coxcombically beneath the nose of a doating Statilia.