Are wilms' tumors malignant?

Yes. Wilms' tumor is a malignant tumor of the kidney that usually occurs in childhood.
Yes. If the tumor involves just one kidney, typically that whole kidney is removed. Then depending on the staging and appearance of the tumor, chemotherapy is used. This chemotherapy can vary from being just outpatient therapy, to chemotherapy requiring hospitalization for high stages. Lastly, some patients will require radiation therapy also.
Yes. Wilms' tumors are cancers of the kidney that occur in children. Surgery is required to remove the entire kidney if disease is confined to one kidney. Oftentimes chemotherapy is also required depending on the size of the tumor and the cell type.