In writing about the racial undertones that dictate concerns on the part of Desiree's parents as well as on the part of her husband, Armand, you may wish to compose a thesis that sets up a comparison/contrast. For, the reaction that Madame Valmonde has to the prospect of Desiree not being white is certainly not the same as the reaction of Desiree's husband, Armaud.
Thus, there is a statement about the nature of love. Madame Valmonde does not waiver in her maternal love for Desiree, the baby that she believed "a beneficent Providence had sent her. For, when Desiree writes to her, she does not confirm or deny anything; she simply tells Desiree to come home. On the other hand, Armaud who "fell in love as if struck by a pistol shot," rejects both his wife and baby. He coldly tells Desiree he wants her to go. Without proof of anything, he rejects her:
...he no longer loved her, because of the unconscious injury she had brought upon his home and his name.
Ironically, it is only after Desiree has gone that Armaud learns the truth.
Are you writing on both topics, or do you have to choose one? If you are discussing the broader subject of racial undertones, your thesis sentence should suggest something about race in the story and should specifically identify some examples of racial undertones. For example:
The rife racial undertones in Chopin's "Desiree's Baby" govern the characters, causing Desiree's adopted parents to worry about her ethnic background, enabling Armand to treat "his" slaves with cruelty, and granting him the power to disown his wife and child.
If you are writing about the narrower topic of irony or the irony of Armand's heritage, your thesis statement should discuss Chopin's use of foreshadowing (which certainly includes the racial undertones) to lead logically to the story's ironic ending.