Using a ruler, measure both of your toddler's feet heel to toe. Since each foot usually differs in size, it is essential to measure them both. If one foot is larger than the other, you should choose the size of the shoe based on the larger foot. Once you have your measurements, use the following shoe size conversion chart to find your child's corresponding shoe size in U. The foot on a tape measure and measure it from the heel to the tip of the longest toe.