https://www.justice.gov/criminal-ceos/citizens-guide-us-federal-law-child-pornography
Images of child pornography are not protected under First Amendment rights, and are illegal contraband under federal law. Section 2256 of Title 18, United States Code, defines child pornography as any visual depiction of sexually explicit conduct involving a minor (someone under 18 years of age). Visual depictions include photographs, videos, digital or computer generated images indistinguishable from an actual minor, and images created, adapted, or modified, but appear to depict an identifiable, actual minor. Undeveloped film, undeveloped videotape, and electronically stored data that can be converted into a visual image of child pornography are also deemed illegal visual depictions under federal law.
You are confusing contraband with intent. It is illegal to have that video, if they do not delete it then they can be charged with a crime. Regardless of whether or not they intended to acquire it, they still posses it. A reasonable person would delete it right away. If they have possession of it and do not realize it, they still could be charged with negligence.