This is not your usual "madlib". This Sillylib exercise is written from a farcical style. Boys tend to especially appreciate the crazy, non-sequitor humor.
If you've done this kind of activity with kids before, you know it's always a good idea to remind students that "school appropriate" is the expectation. I have found kids respectful of that rule. They even come back the next day swapping stories they wrote from the day before.