For some reason, children are very rarely the cosplayers?

Probably because cosplay requires certain financial costs and skills on the preparation of a suit, some "acting" skills, and more. This is still a hobby than a job can pay. Children, probably, if this kind of thing interesting, not the fact that they can afford to collect the costume is good enough, and not the fact that parents (and the child himself earns the money) is willing to invest money in it.
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