In this two-part study, patients are stratified by age (3 months to < 2 years versus 2 years to < 13 years). In Part A, 8 patients (4 per cohort) receive a single intravenous dose of cidofovir with concurrent probenecid. If 1 patient in a cohort experiences life-threatening grade 3/4 toxicity, accrual for that cohort is stopped. If 1 patient in a cohort experiences non-life-threatening grade 3/4 toxicity, 2 additional patients are entered in that cohort. A second dose of cidofovir may be studied in Part A based on the pharmacokinetic and safety data obtained with the initial dose. Patients who complete Part A of the study without serious toxicity may be treated in Part B.