Does every child have a foundling / changeling fantasy? Or just those children growing up in a (narcissistically) dysfunctional family? Or just those children picking up on some level on being donor conceived / adopted?
Whichever it is, I shared a fantasy of this sort with my parents once: "So," I asked smilingly, "isn't it possible that I was confused with another child at the hospital and that I'm not really yours?"
"You're ours," they replied with a stern, concerned look on their faces. "That much is certain. Never doubt it." My mother told me about seeing me for the first time at the hospital and seeing that I had her eyes. This proved it I was theirs.
Not only did I know then that to question this was irrational. Emotionally, I discovered it was somehow unpleasant for them and forbidden for me.