How can i stop my child from thumb-sucking?

Slow extinction: You really can't "make" them stop, but you can start by incentivizing them to stop. Painting fingernails for instance, so they don't want to destroy the pretty image. Or working with them to go perhaps a morning without sucking, and if they do they get a sticker. Get enough stickers and they get a small prize. Usually the social group will get rid of it with peer pressure.

Try various things: There are ingestible substances which ahve an unpleasant odor or taste. You can try using a sock or mitten to cover the thumb at night. You can also have an oral appliance installed.

Teddy bear can work: A toddler (I hope s/he is at least this old) uses the thumb to self sooth while exploring the world.If you give them a teddy bear/soft snuggle of some sort, that will keep one hand/arm busy,it helps them self sooth. Since they need a free hand/fingers to touch and explore, they stay out of the mouth.Don't fuss at them about it, as that gives them anxiety, which requires self soothing.