I've written a parody of a famous children's book, the original being a 60-line poem with illustrations (modern, so definitely within copyright). I've tried to get the parody as close to the poem as possible in terms of meter and rhyming scheme, and the story runs somewhat parallel, but the setting, characters, and overall themes are all polar opposites... there's going to be nearly no overlap in audience. The art style is going to be wildly different, so no risk of confusion there. As best as I can tell, I should have every aspect of Fair Use covered, which is what parody falls under.
The one part where this becomes questionable is in two early lines (lines 4 and 6), which are exact word-for-word copies of the original. This is deliberate, as these lines (and the overall premise) of the original is easily twisted into something very different if the lead character is anything other than a harmless, well-meaning children's book character. My question is, are two lines of a somewhat short poem sufficient to bring "fair use" into question? Or is that a small enough segment that the vast difference in everything else should sufficiently protect me?