Research shows that population-level rates of obesity, which rose dramatically from the 1970s through the mid-2000s, have since plateaued or even started to decline. However, overall improvements may mask differences in trends for different subgroups. For instance, obesity rates have continued to climb among low-income adolescents, leading to growing income-related gaps in obesity. By comparison, we know little about whether income-related disparities have also changed among elementary school children. To address this gap, we examined two cohorts of the Early Childhood Longitudinal Study--Kindergarten cohort, which followed children entering school in 1998 and 2010. We hypothesized that income-related disparities in obesity have also grown larger over time among young children. We used data from nationally representative samples of children who entered kindergarten in 1998 and 2010. We documented rates of overweight and obesity from kindergarten through third grade, examined how rates differed for children from high- and low-income families, and tested whether income-related disparities changed over time. Rates of overweight and obesity were 2 to 5 percentage points higher in the later cohort, and overall increases masked substantial variation by income. Specifically, these increases were driven by children in lower-income households, resulting in substantially larger income-related disparities in overweight and obesity in the later cohort. As we hypothesized, income-related disparities in young children's obesity grew between 1998 and 2014. This suggests that efforts to curb increasing rates of obesity may have been more successful for higher-income families. We discuss potential mechanisms that may account for increasing disparities.