The child of that Chinese spy would be a U.S. citizen. But there’s no need to consider if the child is “natural born” or not.
That’s because of a little-known requirement to be eligible for the presidency: The person must be a United States resident for at least 14 years. So the child would be ineligible based on you saying that he lived his whole life in China.
Side note: Kinda clever for the Founders to throw that requirement in.
Prove he wasn’t here.