Maybe he scribbles on his arms because mummy and daddy are covered in scribble. Maybe he does it because it tickles as the tips run along his soft, bare skin. Maybe, just maybe, he does it because he keeps being told to draw on paper, and paper is just a flat, boring surface that doesn’t tickle and doesn’t look like mummy and daddy. Either way, it’s a step up from drawing on all the walls.
I can think of far worse things to trouble myself over.
For example, flies in my wine.