Previous research reported that positive parenting and parenting stress might impact children’s psychosocial adjustment. The current longitudinal study aimed at evaluating the associations over time between mothers’ positive parenting, their parenting stress, and their preschoolers’ social–emotional competence and emotional–behavioral difficulties. Participants were 53 Italian mothers, aged between 24 and 47 years (M = 35.30, SD = 5.28) at T0, and their children (females = 51%), aged between 3 and 6 years (M = 4.48, SD = 0.84) at T0. Mothers completed self-report scales at 2 time points (with a 2-year lag). An autoregressive cross-lagged model was tested that had a good fit to the data, χ2(6) = 3.37 ns, CFI = 1.00, RMSEA = 0.00. The results showed that maternal positive parenting at T0 negatively predicted maternal parenting stress at T1; maternal parenting stress at T0 negatively predicted children’s social–emotional competence at T1. Moreover, at each time point, children’s social–emotional competence was associated positively with maternal positive parenting and negatively with maternal parenting stress; children’s emotional–behavioral difficulties were positively associated with maternal parenting stress. The results confirm that interactions with mothers are fundamental for children’s psychosocial adjustment. Implications for research and practice aimed at reducing parenting stress and fostering positive parenting are discussed.