The Almighty brought the children of fire into existence long before the creation of the material world and the children of clay. Their number, roughly 400 million, has been fixed since the beginning of time. Contrary to what we might like to believe, angels are a separate species set apart from man. Those good souls who die do not return to Earth as angels spreading peace and love.
There are two main factions of the children of fire--the sons of light and the sons of darkness. The sons of light, who constitute two-thirds of the holy host, still claim allegiance to God. The sons of darkness, comprising the remaining third, show allegiance to Sammael, the accursed one.
The children of fire are immortal, but they are not eternal. Only God is eternal (having no beginning and no end). Still, being celestial creatures makes our concept of death mean nothing to them. They can, however, be destroyed, though this is a rarity indeed. Only extraordinary forces can cause a child of fire to cease to exist entirely. Day to day confrontations between angels and demons are unlikely to produce such results.
An example of angelic destruction occurred at the time the material world was created. When the Almighty proposed the creation of man, the angels of peace and truth were leery of this idea. They made their complaints known to the Almighty, and for this dissention they were destroyed.
In their original inception, the children of fire were to ceaselessly praise God, a duty they joyously fulfilled. With the sin of man and the fall of Sammael, though, their task changed. Many of them were sent out upon the world to help the children of clay achieve salvation and to do battle with the forces of darkness. It is in this capacity that they have passed among us and will continue to pass among us for centuries to come.
The sons of light are not perfection, though. They are inherently flawed beings. The lack of a soul has made them like ancient children. They tend to let their whims take control and often stray from the most righteous path. At the core they are still good, but there is always the chance they may fall. Ironically it is the children of clay, who they were sent to protect, that represent the greatest corrupting influence. The passions and indulgences of the material world are often too much for these celestial creatures to resist.