Among children exposed to elevated maternal depression symptoms (MDS), recent studies have demonstrated reduced internalizing and externalizing problems for those who have attended formal childcare (i.e., center-based, family-based childcare). However, these studies did not consider whether childcare attendance is associated with benefits for the child only or also with reduced MDS. Using a four-wave longitudinal cross-lagged model, we evaluated whether formal childcare attendance was associated with MDS or child behavior problems and whether it moderated longitudinal associations between MDS and child behavior problems and between child behavior problems and MDS. The sample was drawn from a population-based cohort study and consisted of 908 biologically related mother–child dyads, followed from 5 months to 5 years. Attending formal childcare was not associated with MDS or child behavior problems but moderated the association between MDS at 3.5 years and child internalizing and externalizing problems at 5 years as well as between girls’ externalizing problems at 3.5 years and MDS at 5 years. No other moderation of formal childcare was found. Findings suggest that attending formal childcare reduces the risks of behavior problems in the context of MDS but also the risk of MDS in the context of girls’ externalizing problems.