Continuity mistake: When Mrs. McClusky originally comes to the door to tell Andrew about Bree being laid out on the lawn, the cover to the dial for the sprinkler system is open. In the next scene, when Andrew goes over to turn on the sprinklers, the cover is closed.
Continuity mistake: When Bree grabs the twin's toy keyboard, she grabs it and holds it at a downward angle. But in the immediate shot she is suddenly holding it at an upward angle.
Continuity mistake: When Bree is at the restaurant by herself, she is wearing a ring around her neck on a chain which constantly moves position throughout the scene.
Answer: Kayla was sent away in episode 16 (The Gun Song) in season 4, after falsely accusing Lynette of child abuse. She now lives with her grandparents - probably Nora's parents.
I'm curious why we never see her again, why she never talked about, and why Tom never mentions visiting her.