Previous research on autonomic characteristics of social anxiety in children and adolescents has produced highly inconsistent results which may partially be due to task differences and a limited breadth of autonomic measurement. Here we investigated a sample of high (HSA) and low socially anxious (LSA) children, aged 10-12 years before, during and after a standardized evaluated speech task while acquiring a broad set of autonomic and experiential measures. During baseline, we found evidence for tonically higher sympathetic autonomic activity in HSA children, indicated by higher low frequency heart rate variability (LF) and a trend for higher LF to high frequency heart rate variability ratios (LF/HF). In response to the speech task, HSA children showed blunted cardiac responding evidenced by slower increase and delayed recovery of heart rate and a similar significant trend on LF/HF values. Self-reported anxiety, by contrast, showed enhanced reactivity from baseline to anticipation in the HSA compared to the LSA group. The results suggest a restricted cardiac flexibility in HSA children and illustrate that broad autonomic assessment during a well-structured, naturalistic task may improve our understanding of the autonomic physiology of socially anxious children. The results have implications for current theories of social anxiety.