No refund policy for out of stock item

I purchased a pyjamas set for my daughter and when I got home I saw that the top was 2-3 years and the bottom was 3-4. When the items were taken to the store to get the correct size for the bottom, we were advised that the item was out of stock and that we need to take something else for the same value or take a gift card as the store does not do refunds. It is not my fault that the garments were mixed up with the incorrect sizes and nor is it my fault that the store is out of stock. I think this is so unfair on the customers. I need urgent feedback on the matter. The staff should be checking that the correct sizes are on the hangers and not inconvenience the customer. Also the till slip only mentions exchanges within 10 days and nothing about no refunds.