J.E.
you could try putting something on her thumb thats edible but tastes very bad. I sucked my thunb until I was like 10 and I had teeth and jaw problems that I think were caused by it. Its defenitely something you should keep an eye on and try something to make her not want to suck it, but shes still young so she still has time to grow out of it. But I would defenitely try something because once she gets out of the habit she'll probably stop completely. Its just a security thing that makes her feel better when she sleeps.