tell them that something very special takes place between men and woman when they want to create babies but that if they don't do it properly it will leave permanent and lasting scars in their lives and they have to be careful when they allow this very special and pivotal event to happen in their lives. And mention this "special" event repeatedly and only tell them at a later date that it is about sex.
yeah its best if you say stright foward but it would be less awkward if the mother talked to the girl and u talked to the boy depending on how old they are they might already have a good idea of what its about
Give it to them straight, it's just easier. It may be more awkward for you but it'll be easier for them to understand. Personal experiences may not be a good thing to tell 12 year olds or younger, as it may be severely scarring.