Nephron-sparing surgery (NSS) for Wilms Tumor patients has a positive surgical margin rate of 15.7–36.4%. Innovative approaches may reduce the occurrence of positive surgical margins in NSS and prevent these children from having additional radiotherapy and chemotherapy. The feasibility of performing mock surgery on patient-specific hydrogel phantoms of the kidney, tumor, and arterial vasculature for preoperative simulation of NSS was assessed in two patients. The development of patient-specific phantoms allowed the surgeon to practice surgery. Moreover, phantom specimens were assessed using MRI to understand the location and size of the smallest surgical margin. Surgeons reported that simulation surgery helped perform NSS safely and improved intraoperative tumor localization and resection planning. The technique is considered feasible and useful when preparing for NSS. In the future, this technique may further help to achieve negative surgical margins in NSS and may also allow the use of NSS in patients typically regarded as ineligible for this procedure.