Secrets That I Never Want to Know - S8-E1
Character mistake: When Gabby and Bree go to move Gabby's stepfather's car, they realise his car is a stick shift and Gabby says that she has driven a stick shift only once in high school. This is incorrect, as in season 7 episode 8, Gabby has to drive Hector's car home, which was also a stick.
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.