The transition to parenthood has been identified as a significant relationship stressor. Many couples report declines in relationship satisfaction and difficulty with individual stress and co-parenting—problems that have been associated with both child temperament as well as emotional and behavioral problems. Several parenting and relationship interventions have been developed to buffer against these difficulties. In the current study, we report secondary analyses of a randomized controlled trial of brief (6-h) interventions that focused on improving either relationship satisfaction or co-parenting, delivered during pregnancy and the early postpartum period. In this trial, 90 opposite-sex couples (180 participants), who were pregnant with their first child, and were assessed as being at high risk for declines in relationship satisfaction, were randomized to receive either (1) a relationship intervention, (2) a co-parenting intervention, or (3) an information control. At 12 months postpartum, couples who received either the relationship or co-parenting intervention rated their infants as having lower negative emotionality and as having fewer externalizing symptoms compared to the information-only control. Lower externalizing symptoms at 12 months were, in turn, associated with reduced externalizing symptoms at 24 months postpartum. Whereas, lower ratings of child negative emotionality at 12 months were associated with reduced internalizing symptoms at 24 months postpartum. These results indicate that brief relationship or co-parenting interventions delivered during the transition to parenthood have secondary benefits for child mental health.