What surprised me about that conversation was that Harry didn't say something like, "Do you want your parents to find out from you or from one of your or my enemies?" Because it seems to me that the longer Molly holds out the more likely that is how Michael and Charity will find out, even if they don't immediately believe what they hear. Even someone like Ebenezer might stop by looking for Harry; assuming they've patched things up, and accidentally mention Molly's new status in casual conversation.