["Developmental Science, Volume 29, Issue 5, September 2026. ", "\nABSTRACT\nWorking with others is an essential skill for everyday life. Interpersonal neural synchrony (INS) has been suggested as a potential neurobiological correlate of collaboration between parents and school‐aged children. This study investigated whether interpersonal neural synchrony (measured using Wavelet Transform Coherence) is also a correlate of collaboration in young children, whose collaborative skills are still emerging. Data were collected from N = 20 mother‐child dyads with children aged 24–36 months old (mean age 30.15 months) using an fNIRS hyperscanning paradigm. Mothers and their children built Lego Duplo structures collaboratively and individually (separated by a curtain). Dyads showed neural coherence in different regions of the brain when building together and separately, possibly reflecting continued representations of their partner's actions, even when they could not see them. Greater neural coherence was found between the children's left prefrontal cortex (PFC) and the mothers’ right temporoparietal junction (TPJ) while working collaboratively. However, during the Individual condition, coherence was significantly greater between the children's right PFC and the mothers’ left TPJ. Furthermore, dyads were more likely to show overall greater neural coherence during the Individual than in the Collaboration condition if a second researcher was present in the testing session. This study demonstrates how INS can help us understand how mothers and children jointly engage in collaborative tasks. It also suggests a potential influence of others on mother‐child INS, which has not yet been explored.\n"]