If you go to the website of braces manufacturers, they display a warning that "some patients exhibit an allergic reaction to the nickel in the braces". Of course, we are all allergic to nickel. It is a cancer causing poisonous metal.
As the FDA classified it.
I have a daughter who needs braces for an overbite. I am not getting them. I have found orthodontic products that do not use any metal to correct overbites and crooked teeth. Tell them your child is nickel sensitive or is allergic and needs treatment another way. You can request CLEAR plastic retainers! The pink is mercuric oxide usually or cadium. Not good. I would be wary of any dentist the is not aware that stainless steel contains nickel or a dentist that is apprehensive to accomodate you. They are not reading the OSHA sheets for the products they use. Even the manufactures disclose online what is in their plastic and metal products.
Remember...YOU are Paying them!... A LOT OF MONEY to fix your child's teeth. If you want no metal, or clear plastic they should provide that request even if it costs you more money. It is your money and your child's future.
Metal bars are also stainless steel. Personally, I would request a plastic retainer to wear instead.
This is only based upon my research in orthodontics, and I am not condemming anyones decisions or telling you how to treat your children. Just sharing what I found out with research. Buyer Beware!