Yogurt is sour

I have toddler that only eats Parmalat Full cream smooth Strawberry yogurt. Twice a day .Firstly I find the full cream six pack extremely difficult to find in the shops. Some times we have to get the Parmalat Low Fat smooth Strawberry yogurt. The last pack of Parmalat Low fat smooth Strawberry yogurt we bought my son refused to eat. When I tasted it - it was sour. Please now advice us what to do with the stock that we bought and that is sitting in our fridge. I have thrown the receipt away from Checkers so I cannot return it .