The normal temperature inside your child’s body is usually around 37 degrees Celsius. Your child has a mild fever if their temperature is higher than 38 degrees Celsius. A high fever usually means more than 39 degrees Celsius.Fever by itself does not tell you whether your child is seriously sick.our child has a mild fever if their temperature is higher than 38 degrees Celsius. A high fever usually means more than 39 degrees Celsius. Fever by itself does not tell you whether your child is seriously sick.