My name is Germinah and I went to layby clothes for my daughter at the fix last month

My name is Germinah and I went to layby clothes for my daughter at the fix last month. The was one leather jeans my daughter wanted which she took her size but the cashier advised she takes a smaller size cz it stretches. Now I went to take out the clothes on the 3rd and when I got home my daughter tried to fit it but it was small and trying to put it on the tag came off. So today she went to exchange the pants and they said they couldn't cz the tag is out. She has both the tag and the receipt but they still refused yet it was the cashier fault that she got the small pants in the first place. I really don't understand what am supposed to do with a small pants that don't fit cz of a tag that came off while trying to fit it. My daughter took the right size but was advised to take a smaller size now they refuse to exchange it for her.. It's ridiculous that I have to have a small pants which I have no use for cz of a cashier that gave us bad advise.