So then what is the response when someone else's child asks those example questions? If a child asks you if there is a God, do you try to convert them to whatever you believe?
I say what I think. I don't think saying "doesn't seem to exist, but many people believe" may harm any kid.
If a kid asks me of sexual stuff, i'll think twice, because what I say can be transmitted to parents in a different shape.
Let me ask: will you answer "ask your parents" in the same situations? About the religion - do you consciously try to avoid confronting a kid with your beliefs (that may be different)? (just want to figure out)
I don't think it's a question of harm. As for what I'd do, that would depend on the situation. If I answered the question, I would offer what I think, making it clear that that's what I think. But if the child were young enough, and were asking me to find out "the simple truth" rather than what I thought, I would probably tell them it's a question better answered by his or her parents. Especially if it involved specifics like "Is my grandma in heaven?" or "Will I die?"
For the most part, what I tell my son is the truth, though surrounded what I consider to be an appropriate perspective.[1] I think moral authority is important as a parent, and I think that is more solidly gained by telling your child the truth, so that they know they can ask a question of you and you'll give them a straight, true answer, as best you can. Lying about your own foibles, as PG points out, is a "cheap" method of gaining moral authority that backfires badly.
[1] I think a lot of "lies" we tell children have to do with that surrounding perspective. When we lie to avoid discussing certain subjects, it's because a child is not yet equipped to understand enough of the surrounding perspective to make it worth trying to understand the "truth".