Because of having weakened immune systems, children with blood cancers or bone marrow transplants often die from serious fungal infections. Posaconazole is a new antifungal drug that treats more types of fungi with fewer side effects, but we do not know either correct dosing for children or if giving personalized dosing would be better. This study would give information on how to dose posaconazole in children and suggest whether we need to personalize dosing by a child’s genes, age, or whether they have cancer or a transplant.