Another option, how about just reacting to it at the moment when you hear something really jarring.
“Gosh, what a harsh thing to say/ harsh way to put it.”
“They’re dismissing this whole side of the story, that’s not a fair judgement at all”
“Behaving like the way they’re describing is the easiest way to lose friends. Friendships built upon trust and respecting the lines/boundaries of a person. Who wants a friend who does (breach of boundary example)”
"Can you believe this person is making such a big statement without a single proof? "
Keeping the tone casual and at their level should work better. If it sounds like an exam question or job interview, kids would find it difficult to engage. It’s a learning process for adults, too.
Thinking about our own childhood and how we would react to the critical thinking questions should help. Instead of a pop quiz sounding questions, we would prefer the adults talking to us to be genuine and not trying to lecture us, or test us.
You wouldn’t talk to your friend in a way “what’s the streamer’s motivations?” but you’d make a conversation out of it. “I was there with them right until they said this…” And you’d state your reasoning. Think of it talking to your friend, but keep it 12 yo. level.