My daughter is five and still sucks her thumb. She has that same sore. I bought some type of polish to stop nail biting, and I am going to see if that works. My oldest daughter is now 24 ( I have 20 years, alomst, between my kids) and she was a thumb-sucker. She developed the infection and had to have it lanced, drained, and an antibiotic. She still sucked her thumb, and there was nothing I could do about it. She's survived to be 24 and I keep telling myself my five year old will outgrow thumb-sucking, too! I'll let you know how this polish works on my five year old. I am going to try it this weekend.