Expired yoghurt

On the 12th of April I went to buy yoghurts for me and my baby. I bought the Parmalat brand. I had it in my lunch box and my son too. When he came back he had diarrhoea then on Thursday I had another one banana flavour it was very sour when I looked at the expiry date it was the 5th of April. Don't the check their stock before putting it in the fridge. I still have receipt with new and 2 yoghurts with me if you need proof.