Does she already have an overbite? My son sucked his thumb until he was 4.5 and never had an overbite. My daughter is 3.5 and sucks her's and has a terrible one. The dentist said she is just sucking it harder. The only thing that helped my son was to put nail stop (from www.whatshebuys.com). It's the worst tasting nail polish I've ever tasted and it lasts for days. It's so bad that it might make her throw up the first time she sucks her thumb. I tried it with my daughter and she claims to like it. I've spoken to her doctor who suggested we play her game, he feels she does it because we keep saying not to, so he said every so often throughout the day, pull her away from what ever she's doing and place her on the sofa and tell her it's time for her to suck her thumb. If you do this several times a day, she may no longer want to do it.
Now, I've tried that and it doesn't work for my daughter, but the doctor said it works about 1/2 the time.
He suggested I go to the pharmacy and see what they have available behind the counter. He said they have a product for thumb suckers that is suppose to taste really bad, but I haven't had a chance to get it.
I also tried telling my daughter it was okay to suck her thumb only if she is going to bed, but she is constantly sucking her thumb (in bed and not in bed). She just has a very strong will to suck it and no matter what, I think it's up to her to decide when. She claims when she turns 4 (which is in a few months), so we'll see.
good luck. I would try nail stop by www.whatshebuys.com. It's a great product, and very, very bad tasting. Oh, I also placed a drop on her skin right above her nailbed. That way, if she licks her finger, she wont taste it, but if she sucked it, she would.