I would have to take issue with this question and argue that there is nothing in this text to suggest that the baby is being used to symbolise warmth. If we look at the story carefully, we can see that the baby is actually given very little description. She is only described in terms of her relationship with her mother and her father, and the only description we are given of where she is rather than any physical description. The only way that I can see the baby can be used as a symbol of warmth is the impact that its birth has had on its father:
Marriage, and later the birth of his son, had softened Armand Aubigny's imperious and exacting nature greatly. This was what made the gentle Desiree so happy, for she loved him desperately.
The baby then can be used to symbolise warmth in the way in which the story narrates the changed character of Armand, but apart from this I find no evidence from the text to suggest that the baby symbolises any other aspect. The baby in reality is a very minor character who lacks any serious description.