Part B: Children will receive rIL-2 intravenously at the maximum tolerated dose established in part A. Treatment will be given for 5 days every 8 weeks for 3 cycles. [AS PER AMENDMENT 6/4/98: Children will receive rIL-2 intravenously at the lowest dose for 5 days every 8 weeks for 6 cycles. Patients who received this dose in part A will also be offered this regimen.]