Blisters in the mouth of a child are commonly caused by the contagious herpes simplex virus, however, sometimes it can also be caused by stress or lack of rest.
Typically blisters are caused by something burning or irritating the skin which causes it to bubble. The most common culprit of blisters inside of the mouth is eating food that is too hot for the mouth to handle.
Maybe you child drank something really hot, that could have blistered there mouth. Or maybe it is cold sores those can be in your mouth same goes for fever blisters.
