So...my MG character gets bored and colors Abe's lips on a $5 bill. This plays into later helping her to recognize her $ that falls into someone else's hands. The more I've thought about this, though, is it illegal? I mean, I think many children have at one time or another doodled on money. But I don't want to leave this in my ms if it encourages doing something that's considered illegal, no matter how minor it may seem. After looking online, I can't really find a conclusive answer.
I also appreciate any other ideas to make one's money recognizable, without doodling on it.