You don't need Ren'Py-like software for graphic novels unless you want them to be interactive. Even so, it may be difficult for an 8-year-old to learn python and debug something like Ren'Py, especially because half of its use is entirely 18+.

I would recommend something like Scratch. It's a visual scripting language that allows kids to make amazing games and animations. It has all the capabilities necessary with ease and kid-friendly forums and comment sections.