The present study evaluated the possibility of collecting cardiac vagal tone data using a photoplethysmograph, and its stability and continuity in a longitudinal sample of early-school aged children. A method for the optical (i.e., blood volume pulse) estimation of heart rate was established in a pilot study. Then the longitudinal stability and continuity in photoplethysmograph-derived vagal tone was assessed in 114 children (56 girls) at three sessions between 5 and 7 years of age. Results indicate that this method possesses substantial measurement reliability and individual stability, as children report low intra-individual variation over time. Children also report a mean decrease in vagal tone from 5 to 7 years of age, consistent with increased attentional capacity. Overall, this suggests blood volume pulse estimation of vagal tone is both accurate and appropriate for naturalistic developmental research.