When you are talking to young children about sex, do it in a really simple way so that they understand where ‘babies comes from’ and how they as a baby were made. You can talk about this as part of a loving relationship so you do not scare your child.
Use correct scientific words as your younger child is learning new words every day and so will not be confused by some more new ones. There are many books that have been written for younger children that can be helpful with talking about sex and puberty; some are listed on the Preparing to talk about sex page.
As your child gets older, you need to explain to them about puberty and what will happen to their body before it starts to happen, so they can understand it and not be afraid. Do not leave this to the school, as you want to have good communication with your child for when you want to talk to them about HIV, and the school may not always give your child information in the way you want it to be given.
Having these conversations with your child at a young age means that you are creating a relationship with them where you can talk about ‘private’ or ‘family’ issues and where they can ask questions. This will benefit you in many different ways, especially when you decide to talk to them about your HIV.