I will not even ask why you are explaining this to a first grader, since I had my 8th graders make picture books for little kids last year when we performed Macbeth. They weren't allowed to see the play, because of the blood. However, I would explain it as"What's good might be bad, and what's bad might be good, so lets float through the fog and gross air."
Sometimes things are not really what they look like. In fact, something that looks like it is good to eat may actually make you sick. We must be careful to know what something really is before we eat it, or play with it, or even touch it.
For a first grader: "Good is bad. Bad is good. Let's fly through the dirty air."
It is a story of ambition. Those who entertain too much of ambition will get the same plight of Macbeth.