Continuity mistake: When Lynette sits down next to Claire on the bench at the park, Claire is wiping her hands with a tissue. Claire is not wiping her hands in the next shot, but in the shot after that, she is.
Other mistake: Bree is drinking wine with Karl, but before in other episodes and season and episodes and seasons after, she sternly says she does not drink because she was an alcoholic.
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.