Kids dressed as SHADOWS for Halloween - their mother bought black morph suits for them then layered black clothes over those. This may be the scariest thing ever! ^^^ I was a shadow last year, but I just wore all black clothes and a cape.
DIY Peter Pan's Escaped Shadow Costume (or Just a Regular Peter Pan Costume). Would be cute for a duo, one dressed as Peter, one as the Shadow. Or family dad as Peter pan, son as shadow, mom as wendy and add characters as need be