Unidentified Short Story

There was read in my Middle school English class, from a textbook, a short story about a boy who collected a cut-out cardboard model of a castle or fantasy kingdom, in several parts, from the backs of old, maybe antique cereal boxes. I remember him being excited about finding the last part in an old, moldy basement. The plot is that the model transports the boy into the kingdom/castle, and there's a beautiful woman there, I can't remember her role. Now, there was also an old man, who'd apparently also been to the kingdom/castle many, many years before, and had fallen in love with the woman, and when the boy, (who'd been teleporting in and out of the model), tells the old man, he is very excited to see his old love again, and runs with the boy to his home, only to discover that the boy's mother had burnt the model in the fireplace while cleaning. It was read in class to demonstrate that not all stories have happy endings. I still vividly remember my classmates reaction when the story was read, upset disbelief.

The only line I remember is "Oh it was so dusty, I've burnt it on the fireplace." Or something in that vein.