White lie: "Oh no, it's broken."
You will then proceed to tinker around with it under the pretense of trying to fix it, even though you jolly well know that it just needs a battery change.
But your innocent little doe-eyed child doesn't know that, and even if she does, you'll either pretend not to have the right tools to open up the battery panel, or you'll intentionally put in some new batteries the wrong way round to prove that the toy really is broken.
Sounds cruel, but if you have to listen to Elmo's incessant giggling for another 24 hours straight, you truly fear for your own sanity.