This rhyme, like the snowstorm that Winston Smith purchases, functions in a number of different ways. We first are introduced to this popular rhyme in Britain in Chapter 8, when Winston finds the room that he and Julia will use to conduct their affair and also he finds an old man who is selling relics from the time before the rise of Big Brother. Note what the man tells Winston about the rhyme when Winston asks what it was he began to say:
"Oh - 'Oranges and lemons, says the bells of St Clement's.' That was a rhyme we had when I was a little boy. How it goes on I don't remember, but I do know it ended up, 'Here comes a candle to light you to bed, Here comes a chopper to chop off your head.' It was a kind of dance. They held out their arms for you to pass under, and when they came to 'Here comes a chopper to chop off your head' they brought their arms down and caught you."
Thus we can safely say that this rhyme functions in two symbolic ways in the novel. Firstly, it is yet another example of a forgotten world where important facts and childhood rhymes that make up life have been forgotten and erased from the public consciousness. This of course highlights the horror of the totalitarian state where anything that is not propaganda supporting Big Brother is prohibited. Secondly, however, this rhyme foreshadows the fate of Winston and Julia. Note how it ends: "Here comes a chopper to chop off your head." Clearly this rhyme is a clear indication of how Winston's attempts to rebel against the system are doomed to failure.
You might find it useful to compare this rhyme and how it is used symbolically to the snowstorm, which Smith purchases just before hearing this rhyme. Both act as tantalising glimpses of a world that has been lost and forgotten.