.Hi...usually kids do this thumb sucking as a relief seeking behavior or self comfort. This is not a major problem and need not be taken into too much account till 6 years of age. Most children can safely suck their thumb – without damaging the alignment of their teeth or jaws – until their permanent teeth begin to appear that is 6 years of age. Suggestions - 1. Try wrapping the whole palm till wrist if she does it - so that she will not be able to suck the thumb. 2. If she just keeps the thumb inside and doesn t suck vigorously - nothing will happen to the teeth alignment - so do not worry. 3. Punishing her to get her thumb out of her mouth won t help because she probably doesn t even realize she s doing it. Pressuring her to stop may intensify her desire to do it even more. 4. Children usually give up thumb-sucking when they find other ways to calm and comfort themselves. 5.  If you can identify the times and places when your preschooler is most likely to suck her thumb – while watching television, for instance – consider distracting her with a substitute activity, such as a rubber ball to squeeze or finger puppets to play with. 6. Try to praise the kid if she stops thumb sucking at your behest. Hope my answer was helpful for you. I am happy to help any time. Further clarifications and consultations on Health care magic are welcome. If you do not have any clarifications, you can close the discussion and rate the answer. Wish your kid good health. Dr. Sumanth MBBS., DCH., DNB (Paed).