Woolworths contaminated food

We recently bought the Woolworths six pack of 100g yogurts which we always put this into our 5 year old sons lunchbox and also take to work for ourselves. While my son was eating one of the blueberry yogurts at home he complained of chewing something. We kept on telling him it's a blueberry only to finally find out that it is plastic. It is extremely frightening that he has been eating these yogurts daily and for us to tell him it's fine that it is just the fruit. Now we are scared that he could have been ingesting this plastic and we have been oblivious to it because we trust Woolworths food. It is so scary and worrying that I have to inform all parents not to trust this franchise as we just expect them to be good quality. They are definitely not and now we have to take our son to the doctor and spend unnecessary money to go have him checked just so we have an understanding of how bad this is.