As countries develop, the challenge of providing access to the outdoors and nature increases. Consequently, recent environmental justice research has focused on measuring children’s access to parks. The results of these analyses better reflect differences in accessibility, but there are discrepancies between different accessibility models. This study aims to explore child-friendly accessibility measures and proposes a supply–demand-improved two-step floating catchment area (2SFCA) method for estimating urban park accessibility based on children’s needs. The application of this improved 2SFCA method in Changsha City successfully identified areas with unequal park accessibility, offering valuable insights for urban planners, that can be used to promote equitable access to green spaces for all residents, especially children. The results demonstrate that park accessibility in Changsha City exhibits significant differences across various areas, with the lowest accessibility in the western part of Furong District, the northwestern part of Yuhua District, and the southern part of Tianxin District; while the highest accessibility is found in Yuelu District. The limited green space in the central business district of Changsha City, coupled with high population density, indicates a tension between green space planning and population density in the city’s central area. The study proposes that the primary challenge in current green space planning in Changsha is the rational allocation of green spaces to meet the needs of high-density populations within limited urban space. It provides a comprehensive and realistic perspective for understanding the accessibility and availability of green spaces for children, which can help urban planners develop effective policies to support children’s outdoor mobility, while considering equity.