It becomes evident that Roger has been shamed in realizing what he did was wrong. Mrs. Jones recognizes this. She realizes that she has been able to get Roger to realize what he has done as wrong. Mrs. Jones does not further wish to humiliate him or belittle him. She deliberately trusts him with her purse left open and not watching over him. She understands that he does not wish to break her trust. He understands that she could have reported him to the police or even abused him herself. Yet, he has taken him in, to a great extent. She reasoned with him, spoke to him and has told him that she will cook dinner for him. She knows that he does not want to break her trust again. It is for this reason that she does not have to watch over him. She understands fully where Roger is and where his mind is. In not watching over him and leaving her purse open, it becomes clear that he knows she trusts him. If she watched over him while she cooked, he would have taken her for another adult who does not trust him. However, this one is different. This adult does trust him. She trusts him enough to leave her purse near him while she cooks. In this small moment, there is redemption. He experiences this and she knows this which is why Mrs. Jones does not watch over him while she cooks.
Mrs Jones didn't watch Roger while she was preparing dish because she was sure that the boy wouldn't run away as she revealed her kindness in her treatment of the boy.