My oldest daughter (now 41) was sucking her thumb in the womb. She did it up to 3yrs and stopped when we told her if she kept it up her thumb would fall off and she stopped only to start sucking her index finger !! She finally stopped after a couple of months and her teeth grew in nicely.
She’ll quit when she’s ready. My son sucked his two fingers for about 7 years.
I use to suck my thumb my parents bought stop it (in the baby aisle) worked like a charm but it does leave a bad after taste in ur mouth if u try to put ur thumb in ur mouth u can drink something and itll go away
There’s nothing wrong with satisfying herself by thumb sucking otherwise she will be eating too much
I sucked my thumb til I was like 8. Took hardware in my mouth to get me to stop. Only because my dad said it’d cut my thumb off 
I stopped on my own around 4 and my nephew did too! Lol.