Based on the product information (fabric composition and size chart) I ordered size M (one size up to account for shrinkage and growth). Our kids are 5 and 7 years old, 48 and 49 inches tall, weigh 50 pounds each, medium height (not wide down the middle). These pants were the perfect length (ankle length) but the waist was an inch too big. The RBX size guide on Revain says M is suitable for 8 years but the product label says M is suitable for 10-12 years (see image). Overall a good buy but I hope they shrink in width rather than length.
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