[U]sefulness is happiness, and... all other things are but incidental.

Child’s line equates happiness with practical service: to be “useful” (to family, community, reform, or the needy) is not merely a virtue but the core of a fulfilled life. The trailing clause—“all other things are but incidental”—demotes status, pleasure, and even personal ambition to secondary, contingent goods. Read in light of Child’s moral earnestness, the statement frames happiness as outward-facing and ethical rather than inward-facing and hedonistic: contentment arises from contributing to something beyond the self. It also implies a hierarchy of values: usefulness is the stable center; other satisfactions may come, but they should not be mistaken for the main purpose.