As a kid I would sing it happily because it sounded like such a happy song. Then in French class we learned what it actually meant.
“Alouette, gentille alouette. Alouette, je te plumerai.” This means, “Lark, nice lark. Lark, I will pluck you.” The song goes on to name all the different body parts that will be plucked of feathers, including the head, beak, throat, back, wings, feet, tail, and, in some versions, the eyes. Creepy, much?