The Blackwood family has lived in their household for generations, so they are very well-known in the community. However, the novel begins six years after the family was poisoned with arsenic. The narrative is told from the point of view of the the household's only remaining family members: Mary Katherine (Merricat) Blackwood; her sister, Constance; and her Uncle, Julian.
Constance, who is ten years older than Merricat, was tried in court for the poisoning of her family members. Yet, although she was acquitted of the murders, the trial caused much controversy and turned the Blackwood family killings into a kind of urban legend. As such, the children took it upon themselves to mock the situation by creating a song to bother and taunt Merricat:
Merricat, said Connie, would you like a cup of tea?
Oh no, said Merricat, you’ll poison me.
Merricat, said Connie, would you like to go to sleep?
Down in the boneyard ten feet deep!
Since the arsenic that poisons the family was located in the sugar bowl used by the family for tea, the "song" that the kids sing acknowledges this fact. This is also the reason why Constance was the primary suspect of the poisoning. In Uncle Julian's words:
The other dishes used at the dinner were still on the table, but my niece took the sugar bowl to the kitchen, emptied it, and scrubbed it thoroughly with boiling water. It was a curious act.