F.S.
Twins that I care for, 2 years old, both are thumb suckers.
Recently one got an infection in her thumb when it became red and irratated. The infection spread to her mouth.
Antibotics cleaned it up so please keep a close eye on her thumb. It was swollen and red. Now she is sucking her thumb happy as can be. My daughter sucked her thumb and we tried everything to get her to stop including the stuff you put on thier thumb to get them to stop. She would spit it in the sink and suck some more. When she was ten years old her thumb got infected and she did stop. When she was younger, after she started K, we would tell her she could suck her thumb as much as she wanted at home but not at school or in public and that worked. Some babies and kids need more sucking than others.