Out of curiosity, I Googled the phrase, "a parent's worst nightmare."
It generated 764,000 responses. I looked at the first 10 and found concerns that ranged from what you'd expect (rape, murder, life-threatening injury) to inane comic strips and a professional jock influencing children to eat candy. The only duplication among the 10 "worst" nightmares was human sex trafficking.
I draw two conclusions:
1. There's no consensus answer to the nightmare question.
2. Too many writers are ignoring their cliche detectors.
Feel free to add your own thoughts to this theme, or begin a new one.