Unintended pregnancy is prevalent in the United States and has been linked to challenges for both mothers and their children over time. However, studies have not yet identified mechanisms through which pregnancy intention is associated with children's outcomes. Identification of mechanisms among families at risk for negative outcomes will inform early clinical intervention. This current study examined the association between mothers' pregnancy plans reported during pregnancy and children's externalizing, internalizing, dysregulation, and social–emotional competence outcomes at 3 years old among an at‐risk sample of 682 first‐time mothers from the Predicting and Preventing Neglect in Teen Mothers Study. Further, this study identified mechanisms in the association that can be targeted in clinical intervention. Mediation analyses revealed that mothers' parenting stress when children were 24 months old served as a mechanism in the association between unplanned pregnancy and children's low social–emotional competence at 36 months. Implications for research and clinical practice are discussed. Specifically, early intervention with mothers experiencing unplanned pregnancies may help to promote healthy outcomes among their children over time. Copyright © 2016 John Wiley & Sons, Ltd.