Hearing Mozart's "Warnung", K 416 c "Männer suchen stets zu naschen", in a charming interpretation by Ingeborg Hallstein, I remembered this interesting question and subject. The Mozart example bears the subtitle "Einlage (insertion) in ein Singspiel". There, I think, as well as in operettas and musicals may be hided more material. Well I'm not an expert in these genres... But, isn't the role of Cherubino in "Le nozze di Figaro" another example: "Voi, che sapete, che cosa è amor", or is this camouflage? Not about "too young", but playing with it, is: Carl Loewe: "Kind und Mädchen (Child and Girl), op. 62/2, Nr. 5. Highest quality, but far away from "Singspiel" etc. are Franz Schubert's "Blumenballaden": "Viola", D 786 and "Vergissmeinnicht", D 792, and also his song "Die Rose", D 745. This is, what I remember at the moment. But what about the baroque era, English lute songs, and more? Find with the help of accident.