This situation you describe is legitimately one of my greatest fears as a parent. I have little useful advice to give; all I can really say is “good luck” and to research terms like “elsagate,” “gamergate” and “PewDiePipeline” to see what advice actual experts (psychologists etc.) have about deprogramming kids from them.
I was raised allowed to moderate my own content because I was trusted to be intelligent and wise enough to critically select what I watched or read and learn from the mistakes I made if I consumed something negatively influential.
I was raised allowed to moderate my own content because my computer-illiterate parents had absolutely no clue what the Internet was capable of exposing me to. Frankly, it was only dumb luck on their part that I happened to have the right personality and skills not to succumb to inceldom, the Alt-Right, or some other kind of radicalization.
(Even more frankly, maybe I did succumb to radicalization: I am, after all, an urbanist leftist Linux user (among other weird things) who likes to hang out on Lemmy, LOL!)