Pediatric Care eXpansion (PCX) is an ARPA-H program developing nationwide sharing and analysis of pediatric health data across more than 200 hospitals and care centers. Starting with pediatric cancer, the program aims to make clinical, imaging, laboratory, pathology, and molecular information more usable across institutions. Its planned capabilities include structuring difficult-to-use records, privacy- and security-preserving access controls, and near-real-time analysis for decision support and cohort comparison. PCX builds on ARPA-H’s Biomedical Data Fabric Toolbox work and emphasizes open tools, methods, and standards that health systems can use to establish their own data hubs.