A child of a family is any person living in the same household as his or her parent(s) with whom he or she forms a family, if he or she is unmarried and has no spouse or child living in the household. There is no age limit for being a child.
A child in a family can be the child of both parents, of either parent, an adopted child, or a child in the guardianship of either parent. There is no age limit for being a child of a family. A grandson or granddaughter is not considered a "child of a family".
A couple in which all the children have left the parental home is counted as a childless couple.