The purpose of this study is to present a set of empirically derived effect size distributions in order to provide field-based benchmarks for assessing the relative effects of interventions aimed at reducing challenging behavior or increasing engagement for young children with and without disabilities. We synthesized 192 single-case designs that represented data from 162 individuals and nine groups of individuals (e.g., classes) in 53 reports. We generated distributions of standardized mean difference and log-response ratio using 197 effect sizes for engagement and 146 effect sizes for challenging behavior. We examined benchmarks as a function of publication, disability, dependent variable (primary/secondary), and functional relation status and reported distributions separately by engagement and challenging behavior. Overall, the effect size distributions suggest considerable variability in the magnitude of change associated with school-based interventions for engagement and challenging behavior assessed in the context of single-case designs. Data suggest that expected magnitude of change for engagement and challenging behavior interventions may be difficult to predict and that study and effect size characteristics influence the variability of the distributions. Our results have direct implications for researchers relative to assessing the efficacy of interventions aimed at reducing challenging behavior or increasing engagement in young children with and without disabilities.