Why are boys and girls called son or daughter of the commandments?

The terms of "Bar Mitzvah" and "Bat Mitzvah", which is what the asker is driving at come from the idea that up until the point of Bar Mitzvah a boy is blameless for his sins and needs his parents to follow the commandments and atone for him. When a boy becomes Bar Mitzvah, he inherits the commandments and the direct responsibility for his actions. Since he is the youngest person in the room (i.e. no children are in the sanctuary) he is like a new son, the new generation. The concepts apply equally to Bat Mitzvah, but as girls are seen to mature more quickly than boys, the age of Bat Mitzvah is 12 while the age of Bar Mitzvah is 13.