A great lesson I have done to demonstrate this was to create typography self-portraits of children using information about themselves that they would be willing to share online.
Firstly, have the discussion with children why it is important to keep personal information safe and not share it online. List examples of personal information children shouldn't share - full names, names of parents and siblings, address etc.
Ask the children to think about what information they can share - hobbies, football teams, pet names etc.
The app we used to create our self-portraits was the fantastic Type Drawing, which I have used is so many other ways!
Children used all the information they would be willing to share online as the text in their self portraits. Here is an example of what the children created -
Here are some more examples -