The most important things that every preteen should know are that their bodies are going to change.
How and when that might happen is important for them to know. For example, a young lady should know when she is going to menstruate, and if that happens, what she should use for menstruation. She should know about tampons. She should know about feminine napkins. She should know how to protect herself. She should also know that her breasts are going to grow and it may be a little bit painful for them when they start growing.
For males, they should know that their genitalia is going to grow and that they will start ejaculating or having nocturnal emission, or wet dreams, as we call them. That is something that is very normal for them. They are not urinating in the bed. It's something that every single male will go through some time in their life.
They also need to know that this happens for everybody at their own time. Some people may start a little earlier, and some people might start a little later. They also need to know, for males, their voice is going to change. They are going to grow hair in different parts of their body. That's important for them to know. They are going to be some sexual feelings that they are going to have. That's part of growing up and part of the hormones that are coming into their lives.