Objective: Evaluate the validity of the Kids-Balance Evaluation Systems Test (Kids-BESTest) clinical criteria for the Functional Reach Test (FRT) forward and lateral with laboratory measures of postural control in children with cerebral palsy (CP). Design: Psychometric study of face, concurrent, and content validity. Setting: Clinical laboratory. Participants: Children (N=58) aged 7-18 years (ambulant CP n=17, typically developing [TD] n=41). Intervention: Not applicable. Main Outcome Measures: Stability limits in standing were assessed using the Kids-BESTest items for FRT forwards (FRTFORWARD), FRT lateral preferred (FRTLATERAL(P)), and FRT lateral nonpreferred (FRTLATERAL(NP)). Force platforms and kinematic markers were used to collect information on center of pressure (CoP) and joint movement during reach. Analyses included face validity (Kids-BESTest scores compared between CP and TD groups), concurrent validity (agreement between Kids-BESTest scores and digitally derived scores), and content validity (relations between Kids-BESTest scores with kinematic and CoP data). Results: Face validity of Kids-BESTest criteria was demonstrated with lower scores for CP compared to TD groups for FRTFORWARD (P<.001) and FRTLATERAL(NP) (P=.03) and equal scores for FRTLATERAL(P) (P=.12). For concurrent validity, agreement between Kids-BESTest scores and digitally derived scores was good to excellent for FRTLATERAL(both P/NP) (88%-100%) and good for FRTFORWARD (86%-88%) for both groups. For content validity, the CP group Kids-BESTest scores were correlated with CoP-RangeFORWARD during FRTFORWARD (ρ=0.68) and CoP-RangeLATERAL during FRTLATERAL(NP) (ρ=0.57). For kinematic data, correlations were moderate-high between Kids-BESTest scores and range of hip flexion (ρ=0.51) and ankle plantar flexion (ρ=0.75) during FRTFORWARD, and trunk lateral flexion (ρ=0.66) during FRTLATERAL(NP). Conclusion: The FRTFORWARD demonstrated face, concurrent, and content validity. The FRTLATERAL(P/NP) demonstrated concurrent validity, but partial face and content validity. To improve validity of Kids-BESTest FRT criteria, additional descriptors have been added under the scoring criteria to enable clinicians to quantify observed reach strategies.