Thanks for your question. This is a very commonly heard question and I agree that there are similarities between cold sores and impetigo blisters. They look alike mostly. They contain blisters, crust over when they heals and are usually located in and around mouth and mucus membranes associated with mouth. For an inexperienced person it will be difficult to find which type of sore it is.
The basic differences between impetigo and cold sores lie in their origin. Impetigo is caused by bacterial infection. It is seen started as small blisters with fluid filled in it. After some time, a crust is formed and honey colored matter is seen inside it. It is seen developing over the area before it starts healing. Sometimes it breaks open and show reddening of skin with fluid inside the blister leaking out. Moreover, impetigo is not seen only in mouth or facial parts. It can occur anywhere in the body and it is commonly seen in lips and mouth. It is usually itchy by nature. But it will not pain as cold sores. It is important to keep the infected area clean and dry. This infection is contagious and can spread from one person to another. It is not recommended to scratch or touch the area unnecessarily. Also recommend your child to wash hands every now and then. Tropical antibiotics can be used to treat impetigo and the disease is completely curable. Oral antibiotics can simply prevent spread of the infection and it can be eradicated easily by proper medication.
Cold sores are not like impetigo, by origin. It is caused by virus, more specifically, herpes simplex virus. It is the same type of virus which causes genital herpes. The sores are similar to impetigo and looks like small bumps and blisters. They are commonly seen around corners of mouth and such facial areas. They are seen to go away by their own in a couple of weeks. Scabs will be formed after healing and no other symptoms will be seen after healing. They can be spread to other people by touching of the sores, kissing and transmission of body fluids. Sharing utensils, towels etc. also will cause spread of the disease. Cold sores are usually very painful and you cant even bear the pain in the first few days. Some people apply ice to suppress pain caused by cold sores. Some are seen to take over the counter medicines like acetaminophen and Ibuprofen. But these medicines are unable to cure cold sores. Cold sores are viral infections and cannot be treated well. Only symptoms can be suppressed to a little extend using antiviral medication. They are also seen to recur in coming years, once an outbreak occur. Once infected with the virus, it will stay in that human body for a lifetime and causes outbreaks almost once a year or more or less frequently. Stress, menstruation, exposure to sunlight, other mental disorders etc. can cause cold sores to recur. There is no way to completely eradicate or treat cold sores and to prevent them from recurring.
However it is better to understand from your daughter whether she has had any sexual contact with someone. Cold sores virus or HSV can transmit through sexual activities like oral sex and kiss. If that is not the case with her, it is better to consult a doctor so that he can check and confirm the disease. Hope my answer helped. Take care.