Sweetheart, I Have to Confess - S3-E6
Continuity mistake: During the scene with Tom and Nora in the pizza parlour, a wine bottle and a glass are knocked over in an argument. When they pan back to the table as Nora is leaving, the full bottle and the glass are back on the table.
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.