I would not suggest getting too darned formal as others have warned you could get boring.
What I would suggest is that the examples used be things which most or all of the kids already think are false. You might have to find 3 or 4 examples for 2 or 3 different groups of kids and let those who disagree with the issue work together to say why they think it is wrong.
After they have their own attempt at it then you could introduce the various fallacies and errors in thinking that apply to the issue that group has (and a few that don't just to make them work a bit). Let them work with those for a bit to see if they can apply them by themselves.
After everyone has critically analyzed 2 or 3 things they started off disagreeing with then you open it up and let them look at some things they agree with and see if they can be critical of those too.
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