Our team designed an innovative, observation-based motor impairment measure—the Pediatric Stroke Hemiplegic Motor Impairment Scale (Pedi HEMIs). Here we present the results of a survey describing common practices in the pediatric stroke community and the initial psychometric properties of the upper extremity subscale of the Pedi HEMIs (Pedi HEMIs-UE). This is a cross-sectional study whereby participants completed a battery of assessments including the novel Pedi HEMIs-UE. Internal consistency was measured via Cronbach alpha (α). Intraclass correlation (ICC) was used to assess inter-rater reliability (IRR). Concurrent validity was investigated using Pearson or polychoric correlations and simple linear regressions. The study sample consisted of 18 children aged 1.08 to 15 years. Two participants completed two sets of evaluations, totaling 20 data sets. Cronbach α, a measure of internal consistency, was on average 0.91 (range: 0.89 to 0.92). IRR was excellent with the six raters in almost perfect agreement (ICC = 0.91; 95% confidence interval CI: 0.83 to 0.96). Pearson correlation coefficient between the Pedi HEMIs-UE and logit Assisting Hand Assessment (AHA)/mini-AHA was −0.938 (95% CI: −0.979 to −0.827, P < 0.001), indicating excellent concurrent validity. We found excellent feasibility, reliability, and validity of the Pedi HEMIs-UE in a convenience sample of youth with hemiparesis after stroke.