Buying Two Sizes of Kids' Shoes at Once on Zappos

Does anyone actually know what size shoe their child currently wears? Because I absolutely do not, never do, and never will. When it's time to buy new shoes, I buy two sizes at once from Zappos and just send the size that doesn't fit back. It's the easiest thing in the world and the refund is processed as soon as the UPS store scans the package, which means you're not waiting for weeks and following up. My ultimate child-shoe-life-hack.