Katie frowned, looking at her diminished mother. For all intents and purposes on the outside, Kellie looked and was starting to act more and more like a ten year old. A part of Katie felt bad for tricking her mother, lying to her, but in a way, she also felt justified as her own mind was still that of a child even though she was physically mature. She just wanted her mother to relax, to see things her way. The problem was, the moment she grew up and Katie regressed, Katie was afraid that would mean back to normal. So long as she had the ring, she knew she could do what she wanted.
Still, growing up so quickly wasn't quite what she expected, and as fun as driving and stuff were, she didn't want to miss out on her childhood. Maybe if she was a little kid with her mother, she would relax more. Katie reasoned that this was the best option, and so she made her wish.
"I wish I was ten years old, and all the pictures in the house reflected that, and that mom was aware of it..." Katie figured to now make it seem she was aware of her own changes, and thus "pretend" to believe Kellie's own. With this she figured she could ingratiate herself and eventually change her mother back once she was more relaxed and happy. The two could spend some time as kids and just have fun.