Would the police have to show the photos and tape to the parents?

I'm not sure if this helps, but there was an episode of the TV show Criminal Minds where the FBI agents discussed whether to have two sets of parents listen to an audio tape in which a teenage girl begs for her life and then is heard being strangled. One agent says, "Normally, we don't expose parents to such graphic audio or video of a family member's death, but in this particular case we need to identify which girl's voice is on the audio so that we know who was killed." I know it was just a TV show, but that approach by law enforcement would seem to make sense. In the context of this movie, I suppose that the police would only show the photos and tape to the parents if it was absolutely necessary for the purposes of solving the cases of Megan and Amy. I don't see how doing so would help solve the cases of these two girls, so my guess would be that the police would not show anything to the parents.

My friend is a cop and I am a paralegal.

The answer to your question is "it depends."

IIRC, the camera that captured the video belonged to Amy, so technically the video is the property of her parents and they decide what happens to the contents.

If they chose to watch the video, it would be within their legal right to although I am not sure why any parent would want to see their child go through that knowing that the camera is recording the unfiltered brutality of their last moments alive.

The truth is cathartic, but some things are best left unseen and unheard.

That being said, the camera and its contents are evidence of a heinous crime, so the police would petition for its surrender and most likely the parents would comply.

As one of the other posters said, the video would be key in identifying "Josh" so the police would probably capture the audio and canvass around to see if anyone recognized "Josh"'s voice.