Academic scholarship and public discourse about children's digital media use often invokes concepts such as 'screen time' that place the locus of responsibility on individual users and families rather than on designers creating digital environments. In this vision article, we argue that research, design, and policy frameworks that assume individual responsibility contribute to intensive parenting messaging about children's media use, are less likely than systemic approaches to achieve population-level change, and produce inequities in children's access to positive, child-centered media. Platforms (e.g., app marketplaces, video streaming services) act as entry points for children's use of digital spaces, and thus are strong determinants of children's experiences. As such, platforms are an ideal point of intervention for systemic change and have the potential to create equitable and child-centered digital environments at an ecosystem level. We contend that policies that encourage platforms to establish child-centered design as the default user interface will both create better experiences for children and relieve pressure on parents as gatekeepers. Finally, we review the types of research questions that could examine how to measure and optimize platforms for their impact on child wellbeing and outline steps researchers can take to provide evidence-based guidance to industry about designing ecosystems for children's best interests.