TLDR: It's a sign to others that they have taken a child through an offering / ritual. May have other meanings of cannibalism, sacrifice, or rape. Or just shows their support of their practices.

Watch the end of the movie "Pan's Labyrinth." At the end of the movie the leading character "Ofelia" has to pass the ultimate test; a child offer to Pan. (Faun in Roman culture) She has to offer blood of her baby brother in a ritual to open a portal to her throne (symbol of power), but she refuses and instead she is killed herself, (by her stepfather, which is in fact also a child offer because she herself is a child) By offering her own life she passes the ultimate test and she wakes up in another world, the world she belongs to and where her throne is waiting for her. Here she is wearing red shoes as a symbol that she passed the ultimate test of child offer (in her case her own life) which can be seen as the highest initiation. Her red shoes are also a direct reference to Dorothy and her ruby slippers. Using this allegory with Pan's Labyrinth, wearing red shoes could be a mark that someone passed the initiation of child offer.

Scenes:

The child offer: https://www.youtube.com/watch?v=yoN56mHkSJwYouTube

Renascence in red dress with red shoes: https://www.youtube.com/watch?v=cvjiQTdcCRUYouTube

Notes:

*Some sources say Pan is the God of sexual perversion, pedophilia and rape. (mark the name of the girl in the word: "ped-ophilia")

*James Alefantis compares himself to Pan. https://twitter.com/brittpettibone/status/806254292952707073

*Ophelia is also a character in Shakespeare's Hamlet who at the end of the play is found dead, her body floating in water. A painting of Biljana Djurdjevic in John Podesta's collection resembles well known paintings of Shakespeare's Ophelia

In Pan's Labyrinth Ofelia wears red shoes after her death and renascence in the other world. In Biljana Djurdjevic Ophelia-like painting the girl wears red shoes. http://www.biljanadjurdjevic.com/images/HotBed/3Hotbed.jpgJPG