The melody to “Goodnight My Angel” was written long before the lyrics. The original idea was a story about a man who lost his faith. Billy Joel had the lyrics translated into latin and it was to be performed as a gregorian chant.

Then one night while putting his daughter, Alexa Ray, to bed, she asked Billy Joel ‘Where do we go when we die?” This question inspired Billy Joel to change his melody into a lullabye about drifiting off to sleep in the River Of Dreams, which is the name of the next song and title track on the album that ‘Goodnight My Angel’ appears. 

Source: Songfacts