M.,
Around age four - five,you can really start to reason with your kids, they usually just need a good incentive (bribe) and they will quit. I would hold off on the device unless you have already tried different ways of helping her quit.
I like to make a plan together having the "problem solving talk".
I do this with my kids on problems I am not sure how to solve, but that need their cooperation.
Sit her down have a pen and paper. Tell her you have a very important thing to talk to her about and you need her help. Explain the problem and why it is a problem for you and her. Tell her you need her help to figure out how to fix it. Write at the top of the paper the problem. Divide it into two columns. On one side put her name, on the other, yours. (make sure she can see what you are writing and tell her what it says) Then ask her for some ideas, these you write on her side and then tell your your ideas and write them on your side. After you have your two lists, go over them and pick one together that you both agree on. If there is one that you don't like, tell her why you don't think it will work. And if there is one she doesn't like ask her why. (sometimes you might have to pick one of yours and one of hers) Pick a first, a second and a third choice. Tell her you are going to try each one until one works.
Do this also with consequences/rewards if she can help pick the consequence/reward it actually might be more effective.
You will be surprised at what this does to help. It gives her some power and she really will come up with great ideas too.
My oldest sucked her thumb until she was five. At which point we talked about it, (leading up to her birthday) sat down and explained why it was not appropriate anymore, and we both came up with ideas on how to quit and when she would do it. We made our game plan, these are the things we came up with; the week of her fifth b-day, using "no-bite" finger nail polish, using music as a distraction for falling asleep while holding her thumbs inside her hands. She did awesome. The first night she woke up about 2:00am and cried a little and said she couldn't do it, and she wanted to take off the "no-bite". I just held her and told her that I didn't know how to take it off with out it still tasting bad. She resolved to try again, and slept the rest of the night without it. We also prayed together about it, asking for help every day. Once in a great while after that, when she was fast asleep, we would see her sucking her thumb. But never to fall asleep or when she was tired. I never told her, and eventually she stopped that too. She is eight now, and doing great.
I sucked mine when I was little also. And I still have an oral fixation that never seems satisfied! Well its not that bad, but I chew on my nails, not bite them, but they are always near my mouth. And I chew A LOT of gum. So I thought for your little one, you could let her chew sugar free gum as a reward for not sucking her thumb. Then once she has stopped, you could ween her slowly off the gum hopefully in time for school.
good luck with whatever your decide:)
E.
