Love this forum and community of wonderful people.
Thank you Karlat for making this place have a human face!
Re thumb issues: Pressure form parents in young age is less hassle then teens teasing.
I would go ahead and helped her to get rid of this habit sooner then later. Especially feel bad about her hurting thumb. I wonder how strong is the addiction if pain doesn't stop it.
I guess taste and comfort are more important to your daughter now.
I was chewing on pens and pencils when I was about 10-12 y.o. Got rid of that by covering all my pens and pencils with something tasting really bad. It helped.
"All the changes are only in the perceivers." - Baruch Ashlag, 'Shamati, Igrot' http://www.kabbalah.info