It's totally valid that a child dying on-screen is triggering for you. I have PTSD, and that feeling of suspension of disbelief being broken is pretty much exactly what derealization feels like, except I get it about things happening in the real world, as well as in movies and TV shows.

With all of that said, I don't really think you could say the percentage of parents can be said to be causative for how media depicts child death, it's really more of a direct result of the Hays code - prior to the Hays code depictions of child death weren't that rare. Child death was more common in those days too, I suppose. It was after the Hays code fell apart, i.e. the 70s where things started to move towards being more violent, and I think that was more likely caused by mainstream depictions of the Vietnam war on the news - and that was probably the decade with more parents than any other decade prior, given the fertility rate peaked in the 60s.