Purpose: Dyadic synchrony is positively associated with social competence. Although children diagnosed with Autism Spectrum Disorder (ASD) and children diagnosed with Down Syndrome (DS) both have trouble with dyadic synchrony, the origin of their difficulties is fundamentally different. In this mixed method study, we investigated differences in dyadic synchrony and social behavior between children diagnosed with ASD and DS. Methods: Twenty-seven children diagnosed with ASD (10 cisgender females; Mage = 10.98 years; SD = 2.21) and twenty-five children diagnosed with DS (11 cisgender females; Mage = 11.91 years; SD = 2.27) performed a collaborative drawing task with a parent in which they had to synchronize their drawing movements. We continuously tracked their dominant hand movements using wearable accelerometers, and performed Cross-Recurrence Quantification Analysis to extract synchrony measures. Additionally, we compared the social behaviors (interpersonal synchrony, emotion regulation, and social cognition, motivation, and confidence) of these children using quantitative parental questionnaires. Results: Parent-child synchrony measures were significantly higher for children diagnosed with ASD. Yet, parents were significantly more positive about the social behaviors of children diagnosed with DS. No significant correlation between the synchrony and questionnaire measures was found. Conclusion: While children diagnosed with ASD synchronize better during a collaborative task, the social behavior of the children diagnosed with DS (including social synchrony) is more positively evaluated by their parents. Possible reasons for this discrepancy are discussed.