It was a very amusing ending to the story, but the more I think about it the more I wonder if it would be confusing. Maybe it would only be a good, funny ending if you already understood what the phrase meant, because the joke comes in that he wasn't being metaphorical when he asked about the elephant, he was being literal. That could also be a little confusing for younger students who didn't understand the phrase to begin with. On the other hand though, this could be a good introduction to phrases and being metaphorical versus being literal in general and in writing. Another good mini-lesson idea!
Ages 4 to 8 sayith the library and I say yay!