The line comes from that master of the storytelling art, JRR Tolkien, in The Return of the King: “Gandalf! I thought you were dead! But then I thought I was dead myself. Is everything sad going to come untrue? What's happened to the world?" A great Shadow has departed," said Gandalf, and then he laughed and the sound was like music, or like water in a parched land; and as he listened the thought came to Sam that he had not heard laughter, the pure sound of merriment, for days upon days without count.”
I first came across this phrase (outside of Tolkein) in The Jesus Storybook Bible. This makes sense, because the author attends Tim Keller's church. The book is a mixed bag, like most children's Bibles. It's definitely better than anything I had as a little kid, since it begins with a chapter specifically about how most people in the Bible are not heroes to be emulated, because the story is about God and not about role models.
But "everything sad will come untrue" is a motif throughout. So many churches use that as a children's ministry resource that I'm not surprised that it's everywhere now.