I was a thumb sucker until age 5 mostly when I was upset or tired just like your little one. My mum tried everything from dummies, putting hot pepper on my thumb at night to wrapping it in bandages and putting gloves on me. Nothing worked. Finally a little boy at school made fun of me and that broke the habit straight away. Unfortunatly my teeth were horrid by that time and my mum had to spend all sorts of money to have them fixed. I am very stubborn though so that is a worst case senario for you. My lil brother on the other hand sucked his thumb to but the hot pepper worked for him. They now sale bad tasting gels and liquids that are specially designed for puting on childrens hands so that they keep them out of their mouths.
DD was a thumb sucker in the womb and still is now (only at home when very tired) aged 12. We did try to discourage her, but to no avail. TBH I'm not that bothered. It doesn't seem to have ruined her teeth and what other harm can it do?
DS(5) sucks his thumb when he's very tired. Doesn't bother me but His teeth are ok. I don't see a reason for him to stop - it's a way for him to self-soothe and as long as hrs not doing it all day, I don't see a problem.
I think it really depends what sort of sucker they are. Dd 3.4 sucks hers and sometimes it's right up to the palm . Anytime I tell her to take it out she generally does but has also sucked other fingers.
Thanks for your replies! I'll get that book & work on trying to convince her she WANTS to stop sucking it for now... tbh as she only sucks it when going to sleep (or upset), not all day &/or night I'm not too worried about it (yet)