It is normally for a baby or young child to thumb suck, and it rarely a negative thing but when it goes beyond the first few years.  That is because thumb sucking can lead to dental issues. Here are some tips that may help them quit thumb sucking.

Try and limit the amount of time you allow your child to suck their thumb. Such as restrict thumb-sucking to in the house, and not in public. Often times kids thumb suck unconsciously, help your child be more aware of their thumb sucking habit and help find them another way to feel comfort.

Do not use nasty-tasting topical stuff on their thumbs or cover them up with gloves or mitten. Remember they are young children and you do not want them feeling bad in order to brake them of this habit

Redirecting: Whenever you see your child thumb sucking instead of focusing on the thumb sucking try to redirect their attention into doing something else that includes using their hands.  Such as playing with blocks or cars etc.