Perhaps tell them a story and ask them to rewrite it such that the bad guy comes out looking like the good guy and vice versa, or similar manipulations and framing exercises. To pick out manipulative phrases from presidential speeches, like peace, democracy, our great nation etc. But that would directly conflict with what they hear in other classes. Or perhaps use the example of dictatorial propaganda, text and posters alike, point out manipulative stuff.
Perhaps one interesting thing would be to peek behind the curtains. To tell them how news are made, how books are produced, how science works, what is peer review, how they can look up the original primary source (but this is too advanced for kids...). That books and knowledge and articles don't just fall out of the sky, they are deliberately produced with goals in mind.
I fear that ultimately it would devolve into a "don't believe everything you read, kids!", similar to "don't do drugs" lectures.
