My daughter is 7. She asked me once, I basically just told her the truth while focusing on a simple version of how genetics work and skipped the sex part.
It depends on their age, but typically my go to response is "where do you think they come from?" And the answers are usually fairly entertaining. If the parents are all good with me giving an honest answer- I will. I like children to lead conversations though, so I give answers that will only produce more thought and more questions.
Learning from how I learned. I don't know why we don't just tell them. Maybe because the reasons for their child's conception is something the parents don't wish to face, reflect upon or learn from.
"That's a good question. I bet your mother can help you with that."