Measure your child's feet from toe to heel. Then measure from side to side at the foot's widest point. Make sure to measure both feet, as feet are not always the same size.
Once you have measured both the length and width of each foot, note the measurements in inches.
Use a free online shoe size chart to determine your child's shoe size based on the measurements you made. If one foot is a larger size, use that size to determine sock size.
Use the shoe size to determine sock size with a free online sock size chart in order to find the correct sock size.