[tempestn]

I think it depends on how actively parents try to maintain the fantasy. IE how directly they lie. When the kid is ready to know, they'll ask directly. If the parents straight-up lie at that point, then yeah, you can start to have a problem. But you can say, "Well, what do you think?" If they're satisfied with that, they're not ready. But if they say, "I think it's you guys" or whatever, you can ask if they really want to know. And if so, then you tell them the truth (in a nice way), and it'll probably be fine.

On the other hand, it can also be bad if the parent decides it's time for the child to know and tells them before they're ready. (That was what I got from a well-meaning but misguided parent, and it certainly wasn't fun.) Really it's like most 'adult' topics: you can broach the topic and then let the kid show you what level of detail they're ready for with the questions they ask.