If you cannot help the child stop, the orthodontist will have some suggestions. Using a retainer (a removable plastic appliance that fits the roof of the mouth) will usually stop the habit. It changes the feeling of having the finger or thumb in the mouth, and the appliance itself seems to satisfy some of the urge to put a finger or thumb into the mouth. If the habit is more tenacious, addition of a habit breaking guard or a crib to the retainer may be helpful.
Sometimes a fixed brace across the palate can also be used. The appliance will need to be used from six to ten months to make sure the child has stopped the habit. The orthodontist can make some corrective tooth movements with the appliance as well, if indicated.