Introduction Bystander cardiopulmonary resuscitation (CPR) significantly increases out-of-hospital cardiac arrest survival rates. Early CPR implementation and public education are essential for everyone, including children. Despite challenges such as ensuring proper chest compression depth in children, early CPR training at school is beneficial and supported by various studies. However, few studies demonstrate the effectiveness of CPR training for children in environments outside of school, such as health festivals. We conducted chest compression training for children under 15 years who attended a hospital festival and evaluated the changes in their confidence and knowledge of the training. Methods This study evaluated chest compression training in children under 15 years old at a hospital festival. Participants included preschoolers to junior high school students. Training effectiveness was assessed using pre- and post-training questionnaires to measure confidence in and knowledge of chest compressions. Data were analyzed using the χ² test to compare responses before and after training, with a p-value < 0.05 considered statistically significant. The training involved a 10-minute hands-only session using CPR mannequins conducted by paramedics, doctors, and nursing students. Results The study included 180 children (51% male, 49% female). Only 14% had prior CPR training. After CPR training, significant improvements in confidence and knowledge regarding chest compressions were observed among preschool and elementary school students. The overall correct response rate increased from 51% to 97% for the location and from 59% to 92% for the depth of chest compressions. Discussion This study evaluated CPR training for children at a hospital festival and showed that few children have experience with CPR training. Our CPR training improved children's confidence and knowledge of chest compressions. Despite concerns about the strength of younger children in effective chest compressions, preschool, and elementary school children showed high motivation and capability. This study highlights the importance of early CPR education supported by both school and health festival initiatives. The limitations include the short training duration and lack of automated external defibrillator instructions. Conclusion We confirmed that CPR training at a hospital festival improves children's knowledge and confidence. Future large-scale studies are needed to evaluate long-term retention and broader age ranges.