Sucking is a natural reflex. Many children suck their thumbs to soothe themselves. It can help them to feel secure and relaxed, and can even help them to sleep. The problem arises when your child continues to suck their thumb after their permanent teeth start coming in. Doing so can cause issues with the development of their mouth and the alignment of their teeth. Your child should stop sucking their thumb before their permanent teeth begin to erupt. If they are having trouble breaking the habit, there are ways that you can gently help them.