An ideal temperature for viruses to survive and multiply is 37 degrees Celsius. The body's natural response to viral attacks is to raise the temperature to make the living conditions for viruses too harsh to thrive in. That is why temperatures as high as 40 degrees Celsius can be reached when a child is running a fever.
Although a high temperature like that is considered within the normal limits, you should always seek medical attention if you think that their fever is too intense. What can cause brain damage to a sick child, is giving them aspirin or medication containing it. To cool a very high temperature, a warm cloth should be used on the child's forehead, instead of a cold one. The evaporation from the warm water will reduce the temperature gradually. If a cold cloth is used, the child's body will react by raising the core temperature even more.