How to cut a crumbly baked item (e.g. cereal bar) without it disintegrating?
I suggest that you try to go against your habit! :) Cereal bars and the like don't really need to cool on a rack: just put the baking tray in your fridge when you're done baking the bars. That way they'll set much better, as the heated (and thus very soft) sugary components can cool down and firm up. You will find that it's much easier to cut the bars once they've been in the fridge for a while.
It also helps a lot to use a sharp and wet knife when cutting. That way you will slide through the bars more easily and the bars won't stick to the blade of the knife.