All the people don't write accurate ways to write a 9 year old in modern media,
Your complaint that it's crappy writing then a lot of books and shows carry crappy writing, deal with it,
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This, friend, is a fallacy called "whataboutism". Other things being bad don't negate this thing being bad and the two have nothing to do with each other. "It's okay that this is bad if other things are also bad" is a wild take.
Also, plenty of people can write a nine-year-old. It generally has to do with previous contact or basic research. Hope that helps :)