This Sale Is Not Childs-pay

I am checking out a couple who have purchased a few dresses and blouses. I give them their total. 

Customer: “What about the 25%-off sale?”

Me: “That sale is for children’s clothing items only, ma’am.”

Customer: “But these are children’s clothing!” 

Me: “These are adult items, ma’am, in adult sizing.” 

Customer: “But these are for my daughter!”

Me: “Oh, did you need to get something similar to this in the children’s section?” 

Customer: “No! My daughter is twenty-three, but she’s still my child, so give me my discount!”

I try to explain it to her, but she still doesn’t get it. She pays full price but leaves complaining to her husband:

Customer: “But I don’t understand; she’s still our child…”