Teaching a younger reader media literacy works best as an ongoing conversation rather than a single formal lesson, asking questions together while actually reading something: who wrote this, why might they have written it this way, and what would someone who disagreed with it probably say in response.
Comparing two different accounts of the same event side by side is a particularly effective exercise, since it makes the existence of framing and selection visible in a way that no amount of abstract explanation quite manages, letting a young reader notice the gap themselves rather than being told it exists.
The goal is not to make a young person cynical about everything they read, but appropriately curious, someone who instinctively asks a few good questions before accepting a claim, which tends to serve them just as well decades later as it does during the specific story being discussed today. That combination of curiosity and caution tends to serve a reader far better, over a lifetime, than either blind trust or blanket suspicion of everything they encounter.