These Charms or milagros have a darker implication. They are not just symbolic of a prayer prayer or mircle. The practice is combined with a sacrifice. For example if a child has hurt their leg a parent will save money and buy a “Milagro” and give it to the church altar. The more valuable and costly the more likely your prayer is answered. So if a parent buys a gold charm leg and leaves it on the altar of a church then the more likely you’re child will be miraculously healed. This is widely practiced throughout the Caribbean and Central America. I speak from personal knowledge of these customs and can not able to find sauce that directly supported this. Of course the Catholic Churches use these customs to enrich themselve with the gold and silver Milagros that are donated by desperate and poor people but that is how the culture is. It puts those “charms” on the vanderbilt necklace in a very different light. I believe they represent sacrifices maybe human sacrifices.
A similar example : slightly different but the idea is the same.
https://www.google.com/amp/www.chicagotribune.com/sns-guadalupe-mexico-story,amp.html
Bustamante said his own mother crawled to the picture about 35 years ago. "My sister had problems with her pregnancy. When her son was born and everything was OK, my mother promised our Blessed Mother to come on her knees. She had a big problem with her knees for months. But she just said 'I promised I would do it.'"
Like the Zocalo, the site of the shrine has Aztec roots. The Marian apparitions here happened only 10 years after Spanish conquistadors defeated the Aztecs. Temples were torn down and Catholic churches erected on their ruins. A temple to an Aztec goddess, who may have been called the great mother god, once stood atop Tepeyac Hill.
https://zinniafolkarts.com/blogs/news/12956857-what-are-mexican-milagros
Milagros ("milagro" means "miracle" in Spanish) are religious charms that are prevalent throughout Mexico and are traditionally used to request a "miracle" or thank the deities for miracles that have already happened. One can find milagros depicting legs, arms, breast, eyes, cows, pigs, hearts, lungs–just about anything that one would need help with or for which one may have received help. Mexicans purchase milagros that are meaningful to them, attach them to altars, shrines, and sacred objects found in places of worship, and pray for help with illnesses, relationships, animals or just about everything. Remember 99% of Mexico is Catholic. Milagros can be purchased in churches or from street vendors outside of the church.
https://huipilcrafts.blogspot.com/2009/03/milagros.html?m=1
Milagros, also known as an ex-voto or dijes, are religious folk charms that are mostly made in Mexico but are also produced in some other countries of Central and South America. Milagros are an old tradition, used for healing purposes and as votive offerings in Mexico and areas of United States. In Spanish, the word "milagro" means "miracle".
Milagros come in a variety of shapes and sizes, and they might be nearly flat or fully three-dimensional; and they can be made out of gold, silver, tin, or lead.
As part of a religious ritual or an act of devotion, milagros can be offered to a symbol of a saint as a reminder of a petitioner's particular need, or in gratitude for a prayer answered. They are used to assist in focusing attention towards a specific ailment, based on the type of charm used. Milagro symbolism is not universal; a milagro of a body part such as a heart might represent different ideas such as a heart condition, a romance, or any number of other interpretations. Milagros are also carried for protection and good luck!