Normally I will defend Target's return policy, but I am really surprised that they wouldn't do an even exchange for a different size. Was the item on clearance or discontinued?

I think, that since your aunt purchased these outfits, she would be more than happy to supply you with the receipt so that you can make the exchange. If she doesn't have it anymore, hopefully she made the purchase on her credit/debit card. Then Target can look up the purchase to make the exchange or even issue a refund to her card.

If all else fails, maybe you can try someone higher up or the 800 number? I am really surprised that they wouldn't take an even exchange.