Continuity mistake: When Lynette sends Kayla upstairs, her bag is seen both on her shoulder and in her hand between shots.
Continuity mistake: When the boy paints "witch" on Mrs. McCluskey's front door, he holds the container in his left hand and the brush in his right. But when Karen gets out and he runs away with the other kids, the two items switch positions.
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.