In Harry Potter and the Deathly Hallows, Voldemort believes he has killed Harry, and gets Narcissa to verify that. Taking quotes from this post:
‘Is Draco alive? Is he in the castle?’
The whisper was barely audible; her lips were an inch from his ear, her head bent so low that her long hair shielded his face from the onlookers.
‘Yes,’ he breathed back.
Still feigning death on the ground, he understood. Narcissa knew that the only way she would be permitted to enter Hogwarts, and find her son, was as part of the conquering army. She no longer cared whether Voldemort won.
Deathly Hallows - Chapter 36 - The Flaw In the Plan
From the quote, it seems that Harry understood what Narcissa wanted after he revealed Draco was still alive.
How could Harry have trusted her beforehand to not reveal to Voldemort that he was still alive?