Exatecan Mesylate in Treating Patients With Ewing's Sarcoma, Primitive Neuroectodermal Tumor, or Desmoplastic Small Round Cell Tumor

sarcoma, childhood desmoplastic small round cell tumor, metastatic childhood soft tissue sarcoma, nonmetastatic childhood soft tissue sarcoma, recurrent childhood soft tissue sarcoma, localized ewing sarcoma/peripheral primitive neuroectodermal tumor, metastatic ewing sarcoma/peripheral primitive neuroectodermal tumor, recurrent ewing sarcoma/peripheral primitive neuroectodermal tumor, interventional, phase2