Since I don't know the exact 'Little Finger Story', if it's a story about the little finger being small but still important, the moral could be that size doesn't matter. Every part, no matter how small, has its value and role to play.
There might be a family - oriented 'Little Finger Story' where the little finger represents the youngest or the smallest in a family. For example, in a family of fingers (thumb, index, middle, ring and little finger), the little finger might have a story of always being protected by the other fingers, yet having its own unique role, like being able to reach into small spaces that the others can't.
The moral could be that size doesn't matter. Just like the little finger, even though it's the smallest, it has its own unique abilities and importance.
One way could be to take away her privileges like not allowing her to play with her favorite toys for a while. For example, if she misbehaves during playtime with friends, this can teach her that bad behavior has consequences.
One way could be to make her do extra chores. For example, if she's been mean to you, ask her to clean the dishes for a week. It teaches her responsibility.
First, think about her interests. If she likes princesses, start with a princess as the main character. Then, create a simple plot, like the princess trying to save her kingdom. Keep the language easy to understand.
It could be because of some mysterious or creepy event related to the finger. Maybe it's involved in a haunted situation or has an abnormal appearance.