look to the nature and objects of the selections made, the quantity and value of the materials used, and the degree in which the use may prejudice the sale, or diminish the profits, or supersede the objects, of the original work.
The first factor is regarding whether the use in question helps fulfill the intention of copyright law to stimulate creativity for the enrichment of the general public, or whether it aims to only "supersede the objects" of the original for reasons of personal profit. To justify the use as fair, one must demonstrate how it either advances knowledge or the progress of the arts through the addition of something new. A key consideration is the extent to which the use is interpreted as transformative, as opposed to merely derivative.
This isn't passing itself off as the very sought-after Power Rangers short movie everyone's dying to buy; this is a new work that takes the premise of an old work and does something new, with criticism about the whole "child soldier" angle.
Totally fair use.