Background: There is limited evidence on the use of facial nerve function grading scales in acute facial nerve paralysis in children. Objective: To investigate the agreement between and the usability of the House-Brackmann and Sunnybrook scales in children with idiopathic facial paralysis (Bell's palsy) and to compare their ease of administration. Methods: Data from a randomized controlled trial in children aged 6 months to <18 years with Bell's palsy was used. Children were recruited within 72 hours of symptom onset and assessed using the House-Brackmann and the Sunnybrook scales at baseline and at 1, 3, and 6 months until recovered. Agreement between the scales was assessed using the intraclass correlation coefficient (ICC) at each time point and using a Bland-Altman plot. Ease of administration was assessed using an 11-point Likert scale. Results: Comparative data were available for 169 of the 187 children randomized. The ICC between the 2 scales across all time points was 0.92 (95% confidence interval [CI] 0.91-0.93), at baseline 0.37 (95% 0.25, 0.51), at 1 month 0.91 (95% CI 0.89-0.94), at 3 months 0.85 (95% CI 0.80-0.89), and at 6 months 0.96 (95% CI 0.95-0.97). The median score for the ease of administration for the House-Brackmann and Sunnybrook scales was 3 (interquartile range [IQR]: 1-5) and 7 (IQR: 4-8) respectively (P <.001, Wilcoxon signed-rank test). Conclusions: There was excellent agreement between House-Brackmann and Sunnybrook scales, with poorer agreement at baseline. Clinicians found the House-Brackmann scale easier to administer. These findings suggest that both scales can be applied in children.