Any thoughts about how to tell whether a child wants real answers? If it is for the social interaction, what's a graceful way to handle it without squelching real curiosity?

My intuition is that figuring out whether the child wants a "real answer" is not a good use of your time. Instead, you should treat every question as a real question and try to come up with a rule about when to answer them. Reasons to stop answering might include (1) your lack of interest in answering, (2) the answer is beyond the child's ability to comprehend - I can easily image a conversation with a 4 year old that basically resolves to "Go learn calculus," which isn't a useful answer.

And if you decide to stop answering, you can just tell the child that you don't want to answer more questions. If the response to that is "why," then it's pretty safe to say that falsifiable statements are not where the conversation is located.