FWIW, this is what a LaTeX editor might give you. You write paragraphs of text, with some tags to indicate the semantics of the text, and converting that into 'pretty' text on a page is (generally) a second step.
I was more thinking about something like LyX and hide the typesetting engine / markup language for a while. Although when children have experience with an editor like this, supporting them to learn something like HTML (of LaTeX) is probably a lot easier as they understand the concept of structural elements.