Well obviously there is a good answer to this one. If you send a letter saying your daughter is a great fã and how much it would mean to her to send a birthday card and then you send an unfilled birthday card with postage she might fill it out and send it back. I have sent things to many actors and gotten things back. Well Good luck! :)
Well, do you think it really matters for a TWO and a half ano old to get a signature from Donna? I mean, sorry to sound negative but, she is only two and a half. You could always fake a signature from Donna. In reality, I don't think you could get a signature from her but if you do, good luck! Though personally I think a loving cuddle would make a better present for your grandchild than a card with Donna's signature.
