We tried the 18-month and 24-month sizes. The 18-month ones were too small for our (14-month-old), so we went with the 24-month ones which seemed fine but when he starts walking, the problems start. On one of the two feet (not both), the slipper does not stay in place, resulting in him putting more pressure on the inside of his foot (blistering). Obviously, also because they are soft, they easily come off with a lot of movement.