Your child was born with an instinctual desire to suck.
When he was an infant, he would suck on a pacifier or his fingers or thumb to calm and soothe himself. According to Dr. Susan Heitler in "Psychology Today," babies and toddlers who suck their thumbs handled stress better than those who didn’t. However, by the time your child turned 3 or 4, he should have the verbal and emotional skills to handle stress in other ways. By his preschool years, sticking his hands in his mouth is a habit that exposes him to germs and can reshape his teeth.
Distract your child when he sticks his hands in his mouth during the day. For example, if you are playing together and you see his hand head for his mouth, hand him a toy. When both of his hands are playing with a toy, he can’t stick his thumb in his mouth.
Watch your child while she plays alone. When she has gone a minute without sticking her thumb in her mouth reward her without mentioning the thumb. Say something like “You’re playing like such a big girl. I’m so proud of you.”
Read stories together about ending thumb sucking. Ultimately, the decision to stop must be your child’s, but stories that show how thumb sucking spreads germs and can change teeth can help motivate your child to stop.
Stop what you are doing together when your child begins to suck his thumb. For example, if you are reading a story to your child and he begins to suck his thumb, stop reading and sit there. When he pulls his hand out of his mouth, begin reading again as if nothing happened. Never deride or make fun of your child for putting his hands in his mouth.
Help your child recognize the habit at night. Don’t punish or make fun of your child for thumb sucking at night. Instead help your child to see that she has little control over what she does in her sleep. You could say something like “There you are, sound asleep and your finger sneaks into your mouth. That makes it tough doesn’t it?” Brainstorm different ways to remind her to keep her finger out of her mouth. Try covering her hand with an adult tube sock or let her choose a fun puppet to wear to bed.