Impossible - S1-E15
Continuity mistake: Episode 15- When the housewives are sitting on the porch discussing Mike Delfino, Teri Hatcher's green shirt keeps changing positions in the shot. It goes from down around shoulders to close to her neck and back.
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.