Mrs. Miller had initially decided to go along the storyline written in the binder which was seen by Merry about the real identity of the kids. It was clearly mentioned there that they didn't really have a great life before being brought in by the organization.
Well, some really did have worse and clearly their life would not have suddenly start to become better in the future even if they were with their families in Mrs. Miller's perspective. She wanted to persuade or in other words wanted to make them believe she had rescued them.
The organization had rescued them. The powers, the extraordinary powers they had now gave them a purpose in their otherwise useless life.
Mrs. Miller wanted to tell all of them. But first if Merry tells them about everything, it could make her job a little harder. So, Mrs. Miller settled that Merry would live with her for a few days.
