Magic Wishing Stone

So there is a new commercial for the movie "Shorts" which is a movie about 3 kids making wishes on a magic stone. Jonathon of course has seen this, so the other day at a restaurant he asks me the question "What if monsters were real?" to which I responded "then I guess that would make them real." Obviously I am getting a little exhausted with the 'what if' questions as I stated in an earlier post. So Jonathon says "if I had the magic wishing stone, I would wish the world was a dangerous place". I didn't quite understand where he was going with this so I said "why would you want that?" and he says "because then monsters would be real and I really like monsters." So I've finally convinced him that monsters aren't real, thinking that I was saving him from many sleepless nights worrying about them lurking in his closet, when he in fact would love to have monsters around. Also, he seems to be under the impression that although monsters are obviously not real, that he can magically make them be real with a wishing stone. Should I tell him there is also no such thing as a wishing stone or do I let him keep one fantastical thing to believe in?