For significant occasions in a special person's life I like to try to give gifts that they can keep for a long time to remind them of that day. I would suggest a bracelet or necklace, which is something that she can enjoy occasionally as a child and will also treasure as an adult.
They do make very adorable children's jewelry and I still have an engraved silver bracelet that I was given as a child. Of course I can no longer wear it, but it is a nice memento. You could also look for a locket, cross necklace or another pendant of special significance to you that can transition from a small chain for a child to a longer chain when she's older. You don't have to get anything terribly expensive, but you do want something that will hold up to several years of wear. I will say that I was completely enamored of the "real" diamond in a locket that I was given as a child, even though it was only a tiny speck I thought it was the most fabulous thing! I was given a few pieces of jewelry as a child; my mother would let me wear them on "special occasions" and I can still wear them today.
Another idea is to start a charm bracelet and give her a charm every year on her birthday.