I first noticed this on her leg this morning, it goes around her whole leg and is really straight, my first thought was she must of drawn on herself but she's only 3 and couldn't draw that neatly, next thought was maybe she had put a hair band around her leg but just got her out the bath and it's still there
I'm totally baffled as to what it can be and google isn't much help and I've asked her and she's doesn't seem to know either.