You know how ya mess around with some kids about taking their candy... sometimes, diamond-like things that glow without being connected to those extensions coming out of the back of the costume should be a clear indication that... that's not the brightest idea.
If somebody came to my door at halloween dressed as flandre (and a girl of a age that flandre could look) then I'd give them extra candy. Especially if there were dead people and a car on fire.