My eight year old is a tumb sucker. She did it a lot as a baby and toddler. I never said anything and asked my family o please keep their opinions to themselves. Once she started preschool, I told her that she was not allowed to suck it at school. After school she would come home, wash her hands and could then suck it. It helped that this was a rule at her preschool. So she did it less and less over time.
As she has grown older, she has come to realize that it is not socially acceptable to suck her thumb in public and only does it as night in her bed. I have receive mixed opinions from her ped and different dentists about the subject so I just went with what felt right, letting her do it. She is a great child who is happy and well adjusted so if she needs a little extra comfort, it was fine with me.
I would suggest not making an issue over it. Just stress the importance of her washing her hands before sucking her thumb. As she gets older, she will start to realize that she doesn't need that thumb as much.
-Barbara