The Faceless Men can only teach Arya how to change her face, not her entire physique. An 11-year old girl with Jaime's face would not be convincing. It's far more believable for Arya to take Tommen's face to kill Cersei. And considering that the entire Valonqar prophecy was in response to Cersei's question about her children, it's also a lot more consistent.
By the way, it's nice to see someone else from Asia.
Hey, I've got an idea. How about someone have something to say about "The new Daenerys Targaryen"?!!