Hi, I think distract her whenever she does that. Involve her in some activity or the other. Tell her how germs are going inside her and they would grow in her tummy. Sometimes, such gory details about the germs repulse kids and they correct their habit. I used tos uck thumb myself, and left it immediately when my mother scared me saying that your teeth will come out like that of an elephant if I continue to do so. Also, you can treat her if she doesn't suck thumb for a specified time. Eventually you can increase the time of her not sucking the thumb.
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Queen of 2 lovely princess