We describe a rare pediatric case of a phalangeal giant cell tumor of bone with extensive bilateral lung metastases following curettage, wide resection, and amputation. Concurrent peripheral blood eosinophilia and pleural effusion with marked eosinophilia (47%) were present. To discover genetic changes driving tumor metastasis, genomic and transcriptome profiling of the metastatic lung mass as well as germline analysis were performed. Whole exome sequencing detected a histone H3F3A p.G35V missense mutation in tumor cells. RNA sequencing revealed overexpression of receptor activator of nuclear factor kappa-B ligand (RANKL). The patient is alive with no residual disease and uncompromised respiratory function 29 months after amputation of primary tumor and 19 months after surgical resection of his metastatic lung disease.