My daughter has had several pairs of these and they are great for cold weather running, ice skating and winter outdoor exercise. We had several of these when she was 6-9 years old and never had problems with knee cavities. Please note that these are athletic leggings (to absorb sweat and keep the child warm during structured exercise/sport programs). They are very similar to the ones I've used for endurance racing for years and can last for years if washed in cold water and dried (heat can shorten the life of the rubber). They're very durable for structured sports, but spandex gymwear doesn't do very well as casual playwear for a child to crawl into (that's why we stock jeans and inexpensive cotton leggings from Target Cat and Jack).
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