Nice Is Different Than Good - S6-E1
Continuity mistake: At the end of season 5, Lynette finds out she's about 6 weeks pregnant. At the end of the same episode, they do a "Two Months Later." In Season 6, Episode 1, Tom says after an argument with Lynette "Only 8 and a half more months." Lynette should be in her second trimester by then, with at most 6 months to go.
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.