Many YouTube videos to choose from (enter "Marienwürmchen Brahms"). There is a different melody by Schumann, also available on YouTube. Brahms often wrote accompaniments to existing "folk" tunes (for respect, not for lack of inspiration, though many of those tunes were quite recent). I do not know whether this is an example, but it may well be, since he called the collection "Volks-Kinderlieder" and did not count them as an Opus ("WoO 31" - work without work number, number 31). Schumann always invented new melodies.