There is no need to worry about your child’s  thumb-sucking habit during their pre-school years,  however, if they have not stopped by the time that the permanent teeth come in, then life long damage can occur. Many times, peer pressure in school alone can cause a child to stop the habit. However, an occasional child will still persist, especially at night while sleeping. Many methods have been proposed and tried to break the habit. No one technique works for every child. There are positive and negative ways. Using positive reinforcement can be very effective but requires a lot of work. You can get a calendar and stick a star on for everyday that the child doesn’t suck their thumb. So many days in a row can earn a reward. This requires lots of parental participation and is difficult to break nighttime sucking. Other methods include wrapping the thumb in tape or gauze or placing a sock over it while sleeping.