Pen bit Eraser when he was a toothing as a toddler, hence the bite marks Eraser has.

Blocky has a scratch from attempting to throw a water balloon at someone else. He did this up in a tree. Losing his grip, he fell through the branches. All of his scratches completely healed except the visible on. The orangish color is the wood underneath his paint.

The band aids on Pen were also from being kind of obnoxious. I'd imagine him getting them while rolling around in the grass with his siblings. (Pencil and Marker)