The purpose of this study was to demonstrate the usefulness of the Teager-Kaiser Energy (TKE) operator to assess surface electromyographic (sEMG) activity from the hip and trunk muscles during pediatric gait in children with and without cerebral palsy (CP). Muscle activity was recorded from the trapezius, erector spinae, rectus abdominus, external oblique, gluteus maximus and medius, rectus femoris, and semitendinosus bilaterally in ten children with typical development (TD) and five children with CP ages 44.4 +/- 18.6 months. Duration of muscle activity was calculated as a percentage of the gait cycle, and compared to two common onset detection methods, a standard deviation (SD) amplitude threshold method, and the visual inspection from two raters (R1, R2). Relative and absolute agreement was determined using intraclass correlation coefficients (ICCs) and Bland-Altman plots. Of the two automated methods, the TKE method demonstrated better agreement with visual inspection (0.45-0.89) than the SD (0.11-0.76) method. The Bland-Altman plots indicated a smaller bias and 95% confidence interval for the TKE method in comparison to the raters (TKE to R1: -5, 113%; TKE to R2: 4, 95%; SD to R1: -24, 170%; SD to R2: -15, 151%). The use of the TKE operator may better detect sEMG activity in children than the standard amplitude method.