I don't know....I'm kind of disagreeing with this whole thing. I sort of think buying your daughter a gift and then sending the "fake" thank-you is kind of getting MIL off the hook. Sending her the message that it's OK she forgot, because you picked up the slack and your daughter is none the wiser. And it's giving your daughter a false sense that her grandma actually remembered her birthday. I know you're trying to protect your daughter, but are you going to keep covering for Grandma every year. I don't know what you got her, but what if it's something that she "cherishes" from her grandmother and she finds out one day that it was actually you that gave it.
I"m not saying that what you did was wrong because that's a tough situation to be in. I'm just trying to throw in a different angle here. Something to think about.