... but have you considered going down the "What do you think, sweetheart?" avenue?
Actually, that is a good point. Questions like that don't come from out of nowhere. Rather than just handing down wisdom, turn it into a discussion. Not only will you learn why he/she is asking that question, but also it will help you determine what level to place the answer on.
For example, this old joke. A young boy approaches his parent wanting to know, "Where did I come from?" Flash panic at having
The Talk suddenly thrust upon him like that and so early too, he sat down and they went through the whole "birds and the bees" talk. When he was done, his son sat there for a minute looking a bit confused, and then said, "Yeah, but ... . Well, Billy says that he's from San Diego. Where am
I from?"
I am a parent. When my son (now 28) was about 5 or 6, we were watching a DTV video on the Disney Channel (mimicking MTV, they made a series of "videos" setting cartoon clips to popular songs) that had included scenes from
Fantasia of Zeus throwing lightning bolts down on the Bacchanalian revelry. I explained that way back when since people didn't know what caused lots of things in nature like lightning, they created the gods as a way to explain those things. Now, he was a very smart kid (yeah, I am a proud parent, but his psychology-student uncle administered a IQ test as a class assignment and even his prof was surprised at the high score) and he immediately asked me whether the same was true about "God". Yes, people would use God to explain the things in nature that they didn't understand. Then a week or two later at family dinner at my in-laws', my mother-in-law said something like "Thank God" and he sternly corrected her, "God doesn't exist!". Of course, everybody immediately jumped on my case and I don't know whether they believed me that he had reached that conclusion on his own.
So, rather than tip-toe through that kind of a minefield, here's a thought. Make it a discussion with her. "We don't really know" is a good answer, but discuss it with her. Point out that a lot of people do believe in "God", but they don't really know what that is and they all have different ideas about "God".