How to handle an attempted bike theft
Press charges, and have the police issue a warrant to search for a bike similar to yours/ Investigate (Asking her mom/neighbors questions about a bike that may be similar)
If she shows up with one, you're out of luck, but it's probably not likely.
Additionally, there is no way she would be able to pedal away with a bike that doesn't fit her, so her height and physiology may also make your case.
I'd think that when somebody adjusted my seat height, I would notice.
Good luck!