I think drawing can be such a powerful way that we all enhance our communication; how great to be exploring with and supporting this 9 year old. I haven’t done a journal review on the topic (please will you keep us posted if you do?)
For now, just wanted to heartily recommend the Sketchnote Handbook by Mike Rohde. He breaks simple drawings down into their key elements and encouraged people to build their own visual library, which feels relevant to your description. There’s also Eva-Lotte Lamm. Both have some good videos and free resources to help people develop a clear and simple visual lexicon. Could complement your work.