Do you mean the initial scene in the car ? Or the second shot of the aftermath of that scene? Or both?
It was certainly a very upsetting scene - but there is a callback subplot to the aftermath - in that Toni Collette's character visualises the same awful outcome in her son, as happenned to her daughter.
Pretty much any time I think about the parts of the last act that seem a little out of place or tacked on, they end up related to that part. It really sticks out, to me, from the rest of the film.
Anyway, as you said, a touch exploitative, either way. Everyone's line on what's necessary and what's gratuitous is going to be a little different. This was just well beyond mine, I suppose.