"Children of God" are people "born" of God, i.e., by the knowledge and fulfillment of His Will, who have set out on the righteous Path of true Life, "heirs" of His spiritual Goodness and Wealth.

"Children", "Child" is a symbol of a person possessing moral purity, spiritual sincerity, simplicity of feelings, not depravity of character, thoughts, directness of perception, sinlessness of deeds - regarding their conscience.  

"Children" are also people who are spiritually nourished only with someone else's help, spiritually weak, weak to resist the temptations of the flesh and the sinful environment.

Negative Meaning.

"Children" are followers of some false doctrine, dogma, ideology, heirs to idolatry, only formal, Pharisaic service to God.

"Children of fornication" are the followers of the false religions of the world, pernicious political ideologies, cunning philosophies, and various false beliefs, who deliberately pervert and for their own benefit violate the Will of God.