Introduction: Femoral shaft fractures in pediatric patients are treated by elastic intramedullary nailing using titanium or stainless-steel nails. The elastic stable intramedullary nailing behaves as an internal splint, promoting early mobilization. This type of treatment involves a minimally invasive approach, no damage to the growth plates, and no impairment of femoral head blood supply. Purpose: The aim of our study was to identify the negative predicting factors that might lead to an increased complication rate after elastic stable intramedullary nailing of femoral shaft fractures in children. Methods: We conducted a retrospective study on 137 patients with femoral shaft fractures treated by elastic stable intramedullary nailing. Patients’ age ranged between 4 and 17 years. We used data from the medical records of the patients to evaluate postoperative complications. Plain radiographs were analyzed to determine the fracture type, fracture location, and postoperative complications such as delayed union, angular deformities, and limb length discrepancies. Multivariate analysis was conducted to identify predictors for poor outcomes. Results: Complications occurred in 29 patients (21%) and consisted of delayed union, axial deformities, or lower limb length discrepancies. In the group of patients that suffered from complications, mechanism of injury, age, and weight were significant. They were older by an average of 5 years; half of them weighed more than 50 kg and over a half were involved in a road traffic accident. Conclusions: Elastic nailing is a successful tool to treat femoral shaft fractures. Three factors were demonstrated to influence the outcome. The mechanism of injury, age > 11 years, and weight > 50 kg are the most important and are predictors for development of complications such as delayed union or deformity.