I think it also partly depends on your agreement with the couple. If you’ll never see the kid, and it doesn’t bother you, you might not bring it up. But if you will see the kid sometimes, it might be a good thing to disclose. Also, if it bothers you to keep a secret, definitely consider telling. But don’t lie. Also, realize that otherwise great people could be weirded out about that one thing.
I generally think some secrets should come out, and others be kept. The whole complete truth isn’t always the best policy. But that’s an individual situation sort of thing.