Love and sex are very personal things, but children will accept that. Who should talk with whom. A parent of either sex can be an effective sex educator to a child of either sex. Not having all the answers. The message you have sent is that it's okay for your children to ask questions and that you will help them find the answers.
Good opportunities come up in movies, when a neighbor is pregnant, or at bathtime.
Remember that if they are old enough to ask, they are old enough to know the correct answers and the correct words. Be sure that you understand what they are asking. If you are unsure, ask them to explain the question before you answer. Answer the question when it is asked, whenever possible. If you cannot answer at the time, let the child know you are glad that he or she asked the question, and set up a time to answer in full.