<p>Emotion socialization is a dynamic transactional process that unfolds at the moment during parent-child interactions. To better understand these transactions (both parent-driven and child-driven) in early childhood, we conducted a lag-sequential analysis examining sequential contingency between maternal emotion coaching and child emotion regulation at ages 3 and 4 years. Mother-child dyads in the southeastern United States (<i>N</i> = 208 for age 3 timepoint [101 boys, 107 girls] and 227 for age 4 timepoint [115 boys, 112 girls]) participated in a laboratory etch-a-sketch task, which was videorecorded and later observationally coded for maternal coaching of both positive and negative emotions and for child emotion regulation (indexed as compliance, engagement, and low frustration) at 30-s intervals. At age 3, we found two reciprocal sequences: (1) When mothers coached positive emotions, children were subsequently more likely to show compliance, and when children complied, mothers were subsequently more likely to coach their positive emotions; (2) when mothers coached negative emotions, children were subsequently more likely to display frustration, and when children showed frustration, mothers were subsequently more likely to coach their negative emotions. At age 4, we only found parent-driven, positive emotion–related sequences: when mothers coached positive emotions, children were subsequently more likely to show compliance and engagement. Findings shed light on the distinct functions of positive and negative emotions as well as the intricacy of dynamic emotion socialization transactions in relation to child emotion regulation during early childhood.</p>