Always measure both feet.
Most children have one foot larger than the other.
Children's shoes should feel comfortable immediately. Most children don't fully develop an arch until preadolescence and a shoe is not needed to help the arch develop. Children should wear shoes shaped like their foot and that allow plenty of room to wiggle all their toes. Allow a thumb's width from the end of the longest toe to the end of the shoe. The heel of the shoe should fit well and not slide.