The current plan was built around her being disarmingly and appearing as someone who would not be a threat to Jack.
If she were to take on the form of a smaller and younger girl, then she would lose an awful lot of her current physical strength that had been built up in her original body.
When she was in that form, there was not a lot that she could do, and she really would be no threat to a man like Jack.
The only thing that would help her would be her black bandage Evil Spirit, but even that would only have so much effectiveness toward aiding her unless Jack was a part of the supernatural world.
If he had a deeper connection from being exposed to a lot of supernatural events, or if he was a Watcher or a Balancer, then perhaps such an approach would work far more effectively.
