Love these and have tried many brands. They're cotton, but they hold their shape longer than many other brands. They run a bit small so if your child is at the top of the size range I would order the next up.
πͺ 3T Boys Training Underwear: Comfortable and Durable Toddler Underwear
41 Review
Pack of 6 Baby Soft Cotton Underwear for Little Boys - Dinosaur Briefs, Toddler Shark Undies, Children Truck Panties
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Comfortable & Practical City Threads Underwear: Sensitive-Friendly Boys' Clothing
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π¦ ViCherub Boys' Black Large Underwear - Pack of 3 | Boys' Clothing Collection
42 Review
Arctix Youth Pants Reinforced: Large Boys' Clothing for Active Adventures
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Boys' Adidas Active Sports Tricot Athletic Clothing
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π Fruit of the Loom 6 Pair Cushion Ankle Boys' Clothing: Comfy and Stylish Selection for Active Boys
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π Boys' Outdoor Climbing Convertible Trousers - Waterproof | Clothing
47 Review