The Gawker digs this up from a high schooler’s 1999 paper about a childhood camp trip:
I was in the “Herons” group along with about fifteen other 8, 9, and 10 year olds …One Friday in July we went on a trip to Long Beach, Sag Harbor, and Jacob [Bernstein] [son of Carl], Max and I ended up sitting in the sand precociously talking about politics. … At some point, the conversation turned to Nixon and Watergate … During the conversation Jacob told me: “Deep Throat was Mark Felt, he’s someone in the FBI. I’m 100% sure.”
How did Carl Bernstein’s son know? His mom — Nora Ephron — told him. In a blog entry today at Arianna’s blog, Ephron says that even though Carl wouldn’t tell her, she figured it out on her own, and then made a point of telling her kids (!).