Don't let the marshmallows boil. High heat will crystallize the sugars, making the bars hard instead of chewy. Keep the heat just warm enough to melt the marshmallows.
When melting the marshmallows, allow the marshmallows to melt about 95%. The little bits of marshmallow will give the treats pockets of marshmallow. Or add 1 cup of mini marshmallows right at the end before stirring in the cereal.
Gently press the warm crispy treats into the pan, so not press hard, or compact them.