Revealing mistake: In the opening scene with Tom and Lynette, Lynette is standing in the doorway to her house but the interior in the background is different to the interior of her house. there is a blue wall in place where an open plan stair should be. (00:01:00)
Continuity mistake: Lynette climbs on the bed to speak to Tom and is wearing black soled shoes. After they finish talking she is on the bed barefooted. (00:37:34 - 00:38:25)
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.