You really can't stop her from sucking her thumb since you can't take it away. I think I remember reading that permanent damage to the mouth/teeth doesn't become an issue until 5 or 6, so you've got lots of time. Focus on what you can control, which is what she does when you're around. I talked to my daughter at an early age about keeping her hands out of her inside-outside parts in public (inside-outside parts: eyes, mouth, nose, ears, bottom and vulva). These are all the fragile parts that belong to her and her alone, and it's not appropriate for her to have her fingers in any of them in public. (I also told her that no one else can touch these parts of her without her permission, and that if someone tries, she needs to tell me about it.) So, sucking your thumb and picking your nose (for example) are activities for the privacy of your own room. That way she can continue to use the thumb-sucking to settle herself down for a nap or at bedtime, but it won't become an pervasive habit that she turns to whenever she feels uncomfortable (and sucking your thumb at school causes problems!) If it's not an ingrained habit, then it's more likely to fade appropriately away. Putting a lot of energy around it, though, does tend to reduce their ability to stop. Try to be matter of fact with it when she's not in the privacy of her room, just like you are (I'm sure) about asking her not to stick her fingers in her nose in public! (My stepson was strangely encouraged to not just suck his thumb all the time, but to have his finger up his nose at the same time. Neither of his parents ever said a word about it -- you can imagine the response from his peers!) Good luck! And good luck in school, too.