She still hasn't decided what to do. If she doesn't use the card, she's giving Wal-Mart the money. If she uses it, she's getting sucked in. If she gives it away, she's encouraging someone else to go there. Is there anything at Wal-Mart that has intrinsic value and honors American workers? Maybe music CDs....
I know it's not what you asked, but have you considered giving the gift certificate back to the relative and asking for a gift certificate from Target (or some other store acceptable to you) instead? For your daughter's aunt, one store is like any other, so I don't see why she would have a problem with it. For your daughter, it's the difference between having or not having a Christmas present she likes. As an added bonus, your aunt will aunt will send you certificates for the "right" shops from now on, eliminating potential future awkwardness. And who knows? Maybe she'll even ask why
you don't shop at Wal-Mart, in which case you could make your case against Wal-Mart to her. Even though I generally disagree with your objections against Wal-Mart (as you know), I don't see why it would hurt to talk about fair trade, sweatshops, and the like.
Come to think of it, even if your aunt isn't the kind of person you might want to converse with about these things, you can still ask her for a different certificate. "There is no WalMart where we live, but we have X, Y, and Z ..." seems like a perfectly agreeable reason to ask for one.