Can you put a bandaid on her thumb?
I have three fetal thumbsuckers- 3, 5 and 6- My 5 yo just stopped because she wanted a manicure! the boys, however, haven't stopped for any amount of bribery!
Good luck! The bandaids seem to be a deterrent for us- just slows the instinct a bit.
I can vouch for the bandaid thing. I used to suck my finger as a child and when I had a bandaid on my finger I didn't like the way it felt in my mouth so I kept one on there until I finally quit.