Other mistake: When Lynette and Tom are waiting for their kids at the bus stop, the bus that drops off their boys says "Briarcliff Academy." The boys actually go to Barcliff Academy.
Other mistake: Lynette is talking to Susan about Zach's infatuation with Gaby. Susan asks how Zach can afford so many expensive gifts for Gaby, to which Lynette replies that he inherited his grandfather's money when he "turned 18, a month ago." However, in the previous episode, Zach reminds Gabrielle that he is two months older than John Rowland, who, it was mentioned, turned 18 in season 1, two years before.
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.