Objective Autonomic hyperarousal in social situations is considered a genetic vulnerability factor for social anxiety disorder (SAD), but so far it is unstudied in children at risk for developing SAD. We examined autonomic activity during socially stressful tasks in children of mothers and fathers with and without lifetime SAD to reveal possible biological mechanisms of intergenerational transmission of SAD. Methods One hundred ten children aged 4.5 years were asked to sing a song in front of an audience and watch back their performance in the presence of that audience. Heart rate (HR), heart rate variability (HRV), electrodermal activity (EDA), and blushing (cheek blood flow and temperature) were measured in anticipation of, during, and after the tasks. Both parents’ lifetime SAD status was assessed, and both parents reported about their own and their child's social anxiety symptoms. Results Children of parents with lifetime SAD blushed more during the socially challenging tasks than children of parents without SAD. Moreover, children of parents with more social anxiety symptoms showed increased EDA throughout the tasks. Finally, more blushing, increased EDA, and reduced HRV were associated with greater child social anxiety. Conclusions This study adds to the current knowledge on the intergenerational transmission of SAD by providing evidence that children at risk for SAD are characterized by excessive blushing in socially challenging situations. The findings also demonstrate that heightened autonomic activity is a characteristic of social anxiety already during early childhood. Hence, autonomic hyperarousal, and blushing in particular, is likely to play an etiological role in the development of SAD.