There isn't a recipe for this. Use common sense and empathy. When a 6 year old asks you about sex there are limits and a context to their curiosity. Usually th

There isn't a recipe for this. Use common sense and empathy. When a 6 year old asks you about sex there are limits and a context to their curiosity. Usually there's some particular unresolved question in their mind that can be answered truthfully without going into things they're not ready for. You need to understand their mental state. You don't need to pull down an anatomical chart and start talking about sperm. You don't need to clam up and lie or give them the impression the whole thing is an inimaginable eldritch nightmare. You just need to answer the question. (Source: Bringing up kids, since these are the only sorts of questions parents have some special insight into.)