So far, there are issues because child mischief extends to on a computer and now on the internet, so bullying takes place on social media rather than (or alongside) in the school halls, but it seems to me grown ups are afraid of:
~ Kids learning about sex stuff and getting a buzz from looking at porn, especially LGBT+ sex stuff if they’re so inclined.
~ Kids learning about actual history of their societies rather than the propaganda taught in history and government classes, by which they might realize they can’t trust their elders and authorities.
~ Kids learning that no, in fact, there is no clean pathway from school to a functional, satisfying career, and in fact they’re going to be run through half a dozen gauntlets competing with their peers for limited sustenance-income jobs. Lather-rinse-repeat until they wash out, all in support of billionaire vanity projects. And they might realize their society is not raising them in good faith, but to exploit them.