Question: how did tywin not know it was arya?
Answer: Well, one person in Winterfell might know Arya, as he's seen her more recently than her brothers and sisters. ... At the time Arya was working as Tywin's cupbearer. Tywin knew she was a northerner and high-born to boot. But he didn't know she was that northerner or that highborn, or he would never have let her escape.