Kids are such different shapes and sizes! We base our size grade on average measurements (like the charts you might find at the doctor) and try to design with longevity of fit in mind. Our undies are stretchy and the fabric is exceptionally high quality. They will last longer than you think! For styles like tees, elastic-waist pants and sweatshirts, feel free to size up, they look cool oversized. For bundled styles, go with the size you normally get. i.e.,if you child is an average sized 3 year old, a 2-3T should be perfect, but if they are verging on 4 then go with 4-5. When in doubt, size up! Kids grow so fast. :) If you have any questions, feel free to email firstname.lastname@example.org and we'll do out best to help you figure out the right size for your child.