XS should be 5/6. After washing they are more like 2/3. My son is 6 years old, 45 pounds/46 inches and usually wears an A. The shirts barely fit him and when he raised his arms it was a belly shirt. In short, pun - if you buy them, buy a larger size than you need.
Star Wars, Marvel, and Disney Short-Sleeve T-Shirts for Boys from Amazon Essentials
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