C.K.
Hi A.~
Wow! I get to share thumb sucking advice !!
Here goes~ do nothing!
There is nothing you can do to make her quit. It gives her comfort and it is a habit.
It is HER thumb so you can't take it away.
I have 3 children all who were thumb suckers and I was and so was my brother. I had braces on my bottom teeth, not related to thumb sucking and my two older kids sucked their thumbs and had braces but also not related to thumb sucking.
My parents tried so many remedies Cayenne pepper on the thumb, some over the counter nasty tasting stuff, a metal cap that fit over my thumb and tied with ribbon around my wrist. I still sucked my thumb. I quit when I started liking boys (11)
The more you talk to her about it, nag, etc. the more she will think about it and then do it more.
She is very close to the quitting time on her own. (My youngest son had serious emotional issues and sucked his thumb publically until he was 13 years old.) That's another story entirely!
C.