poisoning my children

I guess it could also be oir fault for not checking the expiry date but we have come used to trusting the retailers. We bought a 4 pack ultramel custard from our local p?p express on the 4th of march 2015 and put it in the fridge emmediately when we got home. The following day opened one for my 3 year old son and he ate his. He complained his stomach ached and before my wife gave my 1 year old daughter hers she tasted it and emmediately through up. It was completely sour or rather rotten. Wwhen we returned it my wife was made out a ****. She made them aware that the stock that was still up for sale was already expired and they emmediately took it of the shelve. They refunded us but I still feel betrayed. Will p?p pay for the medical bill should my son have food poisoning?