Children's feet grow at wildly different rates, but here's the average:
Age 1: US 4–5 toddler. Age 2: US 6–7. Age 3: US 8–9. Age 4: US 9–10. Age 5: US 11–12.
Age 6: US 12–13 toddler / 1 youth. Age 8: US 2–3 youth. Age 10: US 4–5 youth. Age 12: US 6–7 youth.
Kids' feet can grow up to half a size every 2–3 months under age 4. Measure every season.
Always leave a thumb's width (about 1.5 cm) of room at the toe. Tight kids' shoes cause foot deformation.