Just FYI from an earlier post: Cold sores are NOT Herpes Zoster. They are normally caused my HSV (Herpes Simplex Virus) type 1 where as HSV type 2 is normally in the genital area and Herpes Zoster is when the Chicken Pox virus resurfaces after the initial chicken pox outbreak. Unfortunately, HSV 1 (cold sores on mouth, nose) and 2 (genital) can swap places on the body in sexually active people and I will let your imagination help you figure out how. :)
A typical Herpes outbreak--any type be it HSV 1, HSV 2 or Chicken Pox--normally has an initial larger episode and then smaller ones in the future. The future outbreak of Chicken Pox would be the Zoster episode.
The Dr. could be right that it is just an abrasion. But if it is a cold sore, she does have it for life. I would not let her share toys/drinks with her sister or others until it is gone--just to be safe. But don't panic, it is just a cold sore and I believe I have read that something like 90% of the population has some form of the herpes virus in their bodies from HSV 1, HSV 2 or chicken pox. They are all herpes viruses.
If the red spot goes away in a day or two it was probably just a bump, abrasion. If it ulsers, it is a cold sore. You can look at pics on-line.
My father had a very mild chicken pox episode as a toddler that did not build up an immunity. As a child he got Zoster. It was so bad that he was confined to a totally dark room for months. It scarred him, damaged his vision, and left the skin on this forehead, scalp and some places on the left side of his body so thin that he can tear through it if he scratches or rubs to hard. At my father's request, when my kids had their Chicken Pox shots, we kept them far away from him to make sure they didn't form any pox blisters that could be transfered to my father.
At that point, I did tons of research to find out about Zoster and chicken pox. If you jump on-line, you can find out anything you want about all, I believe, 8 types of the herpes virus and what each does.