From about 2.5 - 4 yo, our DD9 hammered us with the question, "Is that real?" Refining that boundary between reality and fantasy is what has allowed her to enjoy fiction for what it is. The less realistic a story, the more she can just enjoy it. She can even laugh at elaborate death scenes.
The more possible a story, the less she likes it. Current events are still a no-go.
That is interesting. My son at 3.5 also asks often if something is real or pretend...and has for a year or so. He clearly likes things defined and used to tell us he was pretending before delving into pretend play.
For him, knowing something is pretend and knowing something has a happy ending don't help. I am hoping age and maturity will help.