What causes cold sores and fever blisters in the mouth and throat of children?

Coxsackie virus: Typically causes painful sores in their oral cavities. Can also have non painful blisters on their hands and feet. Hence the name, hand foot & mouth disease.

Herpes virus: Herpetic stomatitis is a contagious viral illness caused by herpes virus hominis (also herpes simplex virus, hsv). It is seen mainly in young children. This condition is probably a child's first exposure to the herpes virus. The sores occur in the mouth and throat.

Virus or bacteria: Cold sores generally appear on the outer lips rather than in the mouth. They are caused by exposure to the herpes simplex virus and stays there for life dormant in one of the nerve tracts. Mouth ulcers inside the mouth may be caused by bacteria or virus. In children this arises because their immune system is not as strong as that of an adult, so they are more prone to outbreaks.