Honesty is the best policy and promptness matters, because that will allow the teacher to make her thank you's to the appropriate party in a more timely manner.
If you have a contact number for her, call her. Just tell her very simply: "Susie, (or Mrs Jones, or whatever you call her), Becky (insert your daughter's name) brought home the thank you note you gave her, but the gift did not come from us. I just wanted to let you know that as soon as possible, so that you would be aware that the person who you intended it for, did not get the note."
If you get a machine or voicemail, leave a message.
You can end it with Merry Christmas or whatever. If you actually speak to her (not a recording) then it will probably evolve into a brief chat full of "oopsie"s and chuckles over Christmas chaos and end with a 'enjoy your holidays' all around.
Don't sweat it, just make the call so she can fix it and the right party will receive the acknowledgement.
:)
ETA: Something as nice as a gift card that was evidently of sufficient value to feed her FAMILY for dinner, definitely should be acknowledged, and if she doesn't, I promise the giver will notice. If it was a $5 Starbucks card, meh... no big deal. But enough to take the family out to dinner? You betcha they'll notice no thank you card.