Unable to do an exchange

I bought clothing at jet town centre two months ago on my account for my daughter who at the time wasn't staying with me, so know that school is starting again she came back home and realized that all the clothes is too small for her. I misplaced my slip but because it was bought on my account and all the tags is still on the garments I took it back to the shop only to find out that the store can't help me without a slip know I'm paying for items that my child haven't even worn and it's unsatisfactory because it's not only one or two items and I don't understand because the clothes haven't been worn