Wwwwaaaahhhhh!!!!!

[hathorcat]

Thats great that you have managed to clear most of it off Guess your kiddo just wanted to demonstrate why we find so many ponies all these years later with pen marks on them

If you have the bulk of it off and its not going to have set too deep into the plastic, I would try and finish up with a magic eraser. That may manage to buff out anything remaining.