Coming Home (a.k.a. I've Got You Under My Skin) - S2-E10
Continuity mistake: When Carlos is about to be vaccinated, him and Gabby sit in the waiting room. Throughout their conversation watch Gabby's arms. At one point she has them down beside her, then in the next she has them folded. In the following shot they are down again, and in the next one they are folded.
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.