ImportanceThe Child Sport Concussion Assessment Tool (SCAT3) is a postconcussion sideline assessment tool measuring symptoms, cognition, and balance in preadolescent children. Minimal normative baseline data exist to aid decision making in clinical and athletic settings.ObjectiveTo collect normative baseline data for the Child SCAT3 in a large cohort of young athletes.Design, Setting, and ParticipantsA cross-sectional study was conducted from May 31 to August 12, 2014, at various sporting events (basketball, soccer, baseball, and swimming) in Central Wisconsin among children 5 to 13 years of age who were English-speaking and did not report a lower leg injury within the past 2 months or a concussion within the past month. Data were analyzed between October 8, 2014, and September 12, 2016.Main Outcomes and MeasuresAll Child SCAT3 components were assessed: child and parent report of symptom number and severity, cognition (Standardized Assessment of Concussion–child version [SAC-C]), and balance (modified Balance Error Scoring System [mBESS] and tandem gait). Summary statistics, mean differences, and effect sizes were calculated for each test component.ResultsParticipants included 478 children (234 girls and 241 boys; mean [SD] age, 9.9 [1.9] years]) and their parents. Age had the largest effect on all Child SCAT3 components, with children 5 to 7 years of age reporting higher mean (SD) symptom severity scores compared with those 11 to 13 years of age (18.2 [10.0] vs 11.3 [9.0]; mean difference, 6.86 [95% CI, 4.22-9.50]; effect size, 0.74) and performing more poorly on the total SAC-C (mean [SD] score, 19.5 [5.1] vs 26.1 [2.1]; mean difference, –6.59 [95% CI, –7.49 to –5.68]; effect size, –2.1), mBESS (mean [SD] score, 1.67 [1.8] vs 0.76 [1.2]; mean difference, 0.91 [95% CI, 0.53-1.29]; effect size, 0.68), and tandem gait (mean [SD] time, 22.2 [8.3] vs 14.0 [3.7] seconds; mean difference, 8.23 seconds [95% CI, 6.63-9.82]; effect size, 1.55). Sex had a small effect on the mean (SD) number and severity of symptoms reported by the child (severity: boys, 15.1 [9.8] vs girls, 11.8 [9.2]; mean difference, 3.31 [95% CI, 1.60-5.02]; effect size, 0.35), mean (SD) number and severity of symptoms reported by the parent (severity: boys, 11.1 [7.7] vs girls, 9.4 [8.1]; mean difference, 1.63 [95% CI, 0.21-3.05]; effect size, 0.21), mean (SD) total SAC-C score (boys, 23.9 [3.9] vs girls, 24.9 [3.5]; mean difference, –0.92 [95% CI, –1.61 to –0.23]; effect size, –0.25), and mean (SD) mBESS score (boys, 1.21 [1.5] vs girls, 0.71 [1.0]; mean difference, 0.50 [95% CI, 0.27-0.74]; effect size, 0.38).Conclusions and RelevanceChild SCAT3 baseline normative symptom, cognitive, and balance scores were different, with a large main effect for age and a small effect for sex. These findings may assist health care professionals with interpretation of Child SCAT3 scores for young athletes with a concussion in athletic and clinical settings.