Using a group-randomized field experimental design, this study tested whether a workplace intervention--designed to reduce work-family conflict--buffered against potential age-related decreases in the affective well-being of employees' children. Daily diary data were collected from 9- to 17-year-old children of parents working in an information technology division of a U.S. Fortune 500 company prior to and 12 months after the implementation of the Support-Transform-Achieve-Results (STAR) workplace intervention. Youth (62 with parents in the STAR group, 41 in the usual-practice group) participated in 8 consecutive nightly phone calls, during which they reported on their daily stressors and affect. Well-being was indexed by positive and negative affect and affective reactivity to daily stressful events. The randomized workplace intervention increased youth positive affect and buffered youth from age-related increases in negative affect and affective reactivity to daily stressors. Future research should test specific conditions of parents' work that may penetrate family life and affect youth well-being.