There are couple problems with making shoe size converter which fits everybody’s needs:
The systems are not fully standardized. The sizes may vary because of differences in manufacturing processes, different allowances, methods of measuring the shoes… It may even differ between countries, though they are using the same Continental European system (Germany, France, Italy). Also, there are several US systems, which vary substantially for sizes far above or below medium sizes.
Children shoe sizes are calculated in different ways. As the young ones are growing very fast, some conversion charts take that growth into account, taking e.g., extra 8% length of the foot. The shoe size is then larger than what would correspond to the actual foot length.
Not only length but also width is important. For wider feet, the larger size may be required.