This is what I could find on what it means at the bottomed is the we'd page I found it from. Hope it helps.
Kids' shoes range from 2C to 13.5C (with "C" denoting child sizes, which fit younger kids) and from 1Y to 7Y (with "Y" denoting youth sizes, which fit older kids). We make a few infant 0C-1C styles, which are typically included in gift packs.
Children's feet grow at individual rates, but you may use the below information as a general guide. To determine your child's shoe size, measure their feet. Infant/Toddler (0 - 4 years old): 0C - 10C Preschool (4 - 6 years old: 10.5C - 3Y Grade School (6 years old and up): 3.5Y - 7Y http://help-us.nike.com/app/answers/detail/a_id/671/~/what-are-the-guidelines-for-kids’-shoe-sizes%3F..