Children with autism spectrum disorder (ASD) participate in less physical activity than the recommended physical activity guidelines. This may be attributable, in part, to community program instructors’ limited knowledge of ASD, specifically, their awareness of the nonverbal expression differences and consequent challenges with understanding these children’s experiences and reactions during program activities. We developed the Exploring EXPRESSions of Autism through Body Language (EXPRESS) observational rating tool to increase awareness of body language communication of children with ASD, and through its use, hopefully enhance the relationship between instructors and children with ASD within community physical activity programs. The purpose of this study was to assess two key psychometric aspects of the 12-item EXPRESS-Code. Three parents of children with ASD participated in interviews to evaluate content validity related to item interpretation (positive/negative body language cue categories). Inter-rater reliability was assessed by having two raters use the EXPRESS-Code to score videos of 26 children with ASD (6–12 years) performing an advanced gross motor assessment. The EXPRESS-Code met the target for content validity with 88.5% agreement on item categorization, although parents recommended renaming the body language categories as ‘engaged’ and ‘not engaged’. Intra- and inter-rater reliability estimates were excellent for the ‘engaged’/ ‘not engaged’ cue categories (ICCs 0.95–0.97). Next steps for the EXPRESS-Code include assessment of the impact of use on: 1) the relationship of a child with ASD and their instructor, 2) instructors’ confidence working with children with ASD, and 3) engagement and enjoyment of the child with ASD within a physical activity program.