I had 5 ounces of breast milk frozen for my son while my husband and I went to a movie. His mother was watching and decided to heat the bag in a boiling pot on the stove. Even though I gave her instructions not to. The top of the bag completely melted on the side of the pot, but she cut into the bag and put the milk in a bottle and fed it to my 5 month old son anyways. Is this ok? He seems fine but I am freaking out a bit
Generally speaking if there was a significantly harmful contaminant you would see signs of a problem fairly quickly.  It is never good to give anyone, especially infants, fluids contaminated with plastics.  Plastics can have hormone like effects or other toxic effects, but these are mostly a problem when exposures occur again and again over time.  Usually a single exposure won't cause serious problems.