Here's a place I found online. They're in the UK bu they do ship to the U.S. I don't know anything about them - but they seem to have motorcycle clothing and gear in leather and textile for kids from age 3 to 15, some with armor.
May be worth a look.
I know what you mean, no one here seems to sell kids/young teenager motorcycle gear that's not moto-cross specific. I have a 15 yo daughter who wears a woman's joe rocket jacket, but it's also too big for her. I may check this out as well.
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