The show is A Wonderful Life, based on the classic film It's a Wonderful Life. Harnick did the lyrics, and the music was written by Joe Raposo, best-known for writing some of the most beloved songs from Sesame Street, including the theme song, "Bein' Green," "Sing," "'C' is for Cookie," and "ABC-DEF-GHI."
This is a song called, "Precious Little," sung in the show by George Bailey. His daughter has asked him to sing her a lullaby, and the number starts out sweet but soon the story he tells begins to intertwine with the disappointments he's feeling in his own life.
Harnick himself sings the song here. It comes from the quite- wonderful 2-CD set, Sheldon Harnick: Hidden Treasures (1949-2013), which you can find here for thems what are interested.